Gas, dust, and star formation in distant radio galaxies

Gas, dust, and star formation in distant radio galaxies
Gas, dust, and star formation in
distant radio galaxies
Gas, dust, and star formation in
distant radio galaxies
Proefschrift
ter verkrijging van
de graad van Doctor aan de Universiteit Leiden,
op gezag van de Rector Magnificus Dr. D.D. Breimer,
hoogleraar in de faculteit der Wiskunde en
Natuurwetenschappen en die der Geneeskunde,
volgens besluit van het College voor Promoties
te verdedigen op donderdag 24 februari 2005
te klokke 16.15 uur
door
Michiel Armijn Reuland
geboren te Groningen
in 1972
Promotiecommissie
Promotor:
Prof. dr. G.K. Miley
Co-promotores:
Prof. dr. W.J.M. van Breugel (IGPP/LLNL & UC Merced, USA)
Dr. H.J.A. Röttgering
Referent:
Prof. dr. M.A. Dopita (RSAA/ANU, Australia)
Overige leden:
Prof. dr. P.T. de Zeeuw
Prof. dr. M. Franx
Dr. G. Mellema
Dr. P.P. van der Werf
voor mijn ouders
But risks must be taken because the greatest hazard in life is to risk nothing. The
person who risks nothing, does nothing, has nothing, is nothing. He may avoid
suffering and sorrow, but he cannot learn, feel, change, grow or live. Chained by his
servitude he is a slave who has forfeited all freedom. Only a person who risks is free.
The pessimist complains about the wind; the optimist expects it to change;
and the realist adjusts the sails.
William Arthur Ward. (1921 - 1997)
Table of contents
vii
Table of contents
Chapter 1. Introduction
1 Theories of galaxy formation
2 Radio galaxies . . . . . . . .
3 Outline of this thesis . . . . .
4 Future prospects . . . . . . .
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5
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Chapter 2. Dust and star formation in distant radio galaxies
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Sample Selection and Observations . . . . . . . . . . . . . . . . . . . . . . .
2.1 SCUBA photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Potential contamination of the thermal submillimetre flux . . . . . . .
2.3 Dust template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Observational Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Correlations between parameters . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Redshift dependent submillimetre properties . . . . . . . . . . . . . .
5.1.1 Flux density and relative detection fraction . . . . . . . . . . .
5.1.2 Investigation of the difference between detections and nondetections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3 The increase of submm luminosity with redshift . . . . . . . .
5.2 The connection between submm and radio luminosity . . . . . . . . .
5.3 An anti-correlation between submm flux and UV polarisation . . . .
5.4 Submillimetre and Lyα flux . . . . . . . . . . . . . . . . . . . . . . . .
5.5 L850 and linear size . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Submillimetre and near-IR emission . . . . . . . . . . . . . . . . . . .
5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 A comparison of radio galaxies with QSOs . . . . . . . . . . . . . . . . . . .
7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 3. An obscured radio galaxy at high redshift
1 Introduction . . . . . . . . . . . . . . . . . . . . .
2 Observations and Results . . . . . . . . . . . . . .
2.1 Selection and Keck Imaging . . . . . . . . .
2.2 JCMT and IRAM Observations . . . . . . .
2.3 Keck Spectroscopy . . . . . . . . . . . . . .
3 Discussion . . . . . . . . . . . . . . . . . . . . . .
3.1 Identification and Redshift Estimate . . . .
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Table of contents
viii
3.2
Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Observations and Results . . . . . . . . . . . . . . . . . . . . .
2.1 SCUBA photometry . . . . . . . . . . . . . . . . . . . . .
2.2 IRAM Photometry . . . . . . . . . . . . . . . . . . . . . .
2.3 Chandra . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 Redshift estimates . . . . . . . . . . . . . . . . . . . . . .
3.2 X-ray properties . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Radio–X-ray relation . . . . . . . . . . . . . . . .
3.2.2 Submm–X-ray relation . . . . . . . . . . . . . . .
3.2.3 Obscured nucleus . . . . . . . . . . . . . . . . . .
3.3 Young starbursting radio galaxies . . . . . . . . . . . . .
3.4 Implications for Type II AGN and XRB . . . . . . . . . .
4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 5. The influence of ISM characteristics and AGN activity on the far
infrared spectral energy distributions of starburst galaxies
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 The Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 ULIRGs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 High redshift sources . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Infrared emission models and fitting procedures . . . . . . . . . . . . . . .
3.1 Starburst Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Dust and PAH implementation . . . . . . . . . . . . . . . . . .
3.1.3 ISM pressure and molecular cloud dissipation timescale . . . .
3.1.4 Visual extinction . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 AGN torus models . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 The fitting procedures . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 General Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Comparison with literature . . . . . . . . . . . . . . . . . . . . . . . .
5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Limitations of the models . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Molecular cloud dissipation time scale . . . . . . . . . . . . . . . . . .
5.4 AGN contribution to MIR and FIR wavelengths and its influence on
the inferred star formation rates . . . . . . . . . . . . . . . . . . . . . .
5.5 Star formation at high redshift . . . . . . . . . . . . . . . . . . . . . . .
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
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Table of contents
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Chapter 7. Metal enriched gaseous halos around distant radio galaxies
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Observations and data analysis . . . . . . . . . . . . . . . . . . . . .
2.1 Sample Selection . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Optical and Near-Infrared Spectroscopy . . . . . . . . . . . . .
2.2.1 LRIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 ESI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 NIRSPEC . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 Notes on individual objects . . . . . . . . . . . . . . . . . . . .
3.1.1 4C 41.17 . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 4C 60.07 . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 B2 0902+34 . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Ionizing source . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Central region . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 Extended region along the radio axis . . . . . . . . . . .
4.2 Radiative transport: to scatter or not to scatter . . . . . . . . .
4.3 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 General kinematic structures . . . . . . . . . . . . . . .
4.3.2 Outflows . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 A comparison with CO observations . . . . . . . . . . . . . . .
4.5 Metalicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Mass estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4
5
Introduction . . . . . . . . . . . . . . . .
Sample Selection and Observations . . .
2.1 Sample Selection . . . . . . . . . . .
2.2 Keck Imaging . . . . . . . . . . . .
2.2.1 Lyα imaging . . . . . . . . .
2.2.2 Broad-band imaging . . . .
2.3 HST Imaging . . . . . . . . . . . . .
2.4 Relative Astrometry . . . . . . . . .
2.5 Continuum subtraction . . . . . . .
Results . . . . . . . . . . . . . . . . . . .
3.1 4C 41.17 . . . . . . . . . . . . . . . .
3.2 4C 60.07 . . . . . . . . . . . . . . . .
3.3 B2 0902+34 . . . . . . . . . . . . . .
Discussion . . . . . . . . . . . . . . . . .
4.1 Cooling flows and radio lobes . . .
4.2 Starburst superwinds . . . . . . . .
4.3 Radiation pressure driven outflows
4.4 Obscured AGN . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . .
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Table of contents
x
5
4.6.1 Luminosity based masses
4.6.2 Dynamical mass estimates
4.7 Cosmological Implications . . . .
Conclusion . . . . . . . . . . . . . . . .
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Nederlandse samenvatting (Dutch summary)
143
Publications not included in this thesis
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Curriculum vitae
153
Nawoord / Acknowledgments
155
Chapter 1
Introduction and summary
R
galaxies are some of the most energetic and largest galaxies in the Universe.
They are presumed to be powered by accretion of material onto super massive
black holes at their nuclei and in the early universe they are embedded in spectacular
gaseous emission line nebulae. This makes them interesting objects in their own right.
Further, they provide unique insights into the formation and evolution of galaxies.
The aim of this thesis is to obtain a better understanding of galaxy formation and
evolution, by studying the connection between star formation and nuclear activity in
radio galaxies. To achieve this, we have undertaken a large observational program to
collect data for these galaxies, to obtain information on gas, dust, and star formation
over a large range of the electromagnetic spectrum, from X-ray to radio waves.
The galaxies in this program provide data about their evolutionary state at a time
when the Universe was approximately 10–20% of its current age (13.7 billion years
according to current estimates). The next sections summarize current ideas about
galaxy formation, describe key properties of radio galaxies, show why radio galaxies are uniquely suited for studying the process of galaxy formation, and set the scene
for presenting the results of this thesis.
ADIO
1 Theories of galaxy formation
Galaxies are usually classified into three main classes: the beautiful spiral galaxies,
and the visually less appealing (but no less interesting) lenticulars and ellipticals. If
galaxies cannot be classified in one of these classes, they are called irregulars. The spatial distribution of galaxies appears bimodal. Many galaxies are isolated, the so-called
field galaxies. Others are gathered in groups or clusters. Regardless of their specific
distribution, they are separated by large distances with little material in between. The
tiny (of order 1:100,000) fluctuations in the cosmic micro wave background, that is
thought to be the afterglow of the Big Bang, indicate that the early universe must have
been very smooth. This is in stark contrast to the present clumpy distribution of matter. A fundamental question that now arises is: How and when did the initial density
fluctuations amplify and form the galaxies, groups, and clusters observed in the local
universe?
There are two main scenarios that attempt to answer this question. The build-up of
galaxies could occur through monolithic collapse or through hierarchical merging. In the
monolithic collapse scenario cooling of a single gas cloud is envisaged to result in an
entire galaxy (e.g., Eggen et al. 1962). One feature of this scenario is that it can explain
1
2
Chapter 1. Introduction
the presence of massive galaxies already in the early stages of the Universe. In contrast,
the hierarchical Cold Dark Matter (CDM) theory of structure formation predicts that
the formation of galaxies is a gradual and biased process (e.g., Toomre & Toomre 1972;
White & Rees 1978). In this scenario larger objects grow from the peaks in the initial
mass fluctuations through the merging of smaller, younger objects. The most massive
objects are expected to form at the centers of over-dense regions which will eventually
evolve into the clusters of galaxies seen today. Initially, all objects would be very gas
rich with relatively few stars and their dynamics would be dominated by the CDM
halos. The merging process may strongly affect these early galaxies. It would enhance
starburst activity in interacting systems and accretion onto a growing galaxy at the
center may fuel central massive black holes.
The scenario of hierarchical structure formation is supported by observations of
both local and distant galaxies. Locally, nearly all ultraluminous far infrared galaxies
( LFIR > 1012 L ; for a review see Sanders & Mirabel 1996) appear to be mergers. It is
thought that their large far infrared luminosities are due to dust heated by radiation
from vigorous starbursts and black holes that are being fed efficiently.
In the distant universe, Hubble Space Telescope observations of radio galaxies show
that they have clumpy morphologies (Pentericci et al. 1999). The individual clumps
are reminiscent of smaller but still massive galaxies (Lyman break galaxies; Steidel et
al. 1996) that are expected to merge with the central galaxy on dynamical timescales of
108 years (Pentericci et al. 1998). Near-infrared studies also suggest that radio galaxies
evolve from “fuzzy” structures with large-scale diffuse emission at early epochs into
more compact objects by z ∼ 2 (van Breugel et al. 1998).
A fundamental prediction of CDM theories is that massive galaxies form at the
centers of clusters. Recent observations of radio galaxies support this prediction (Venemans et al. 2002; Miley et al. 2004)
There appears to be a tight correlation between the masses of the stellar bulges
of galaxies and their central black holes (Magorrian et al. 1998; Gebhardt et al. 2000;
Ferrarese & Merritt 2000). This is an issue that a theory of galaxy formation should
explain.
Furthermore, while models of galaxy formation seem to produce the observed number of intermediate-mass galaxies, they overpredict both the numbers of very massive
and low-mass galaxies. Because stars can form only from cold gas, the temperature
of gas is a crucial parameter. Gas is easily heated by radiative and mechanical feedback processes (e.g., UV radiation from stars, supernovae, mergers or radiation and
outflows from massive black holes). Feedback, therefore, could possibly resolve the
mismatches between observations and theory, but its precise role and the main agents
need to established.
In light of the big question “When and how did galaxies form?”, the following
unsolved questions are important:
• What is the origin of the relation between bulge mass and black hole mass?
• Which physical processes control the observed shape of the galaxy luminosity
function?
• Do all massive galaxies reside in (forming) clusters ?
Studies of radio galaxies can help to answer these.
Section 2. Radio galaxies
3
2 Radio galaxies
The research presented in this thesis addresses different aspects of galaxy formation
through studying some of the most extreme objects in the Universe: radio galaxies.
In the nearby universe, radio galaxies are hosted by very massive (M ∼ 1012−13 M )
ellipticals. Below, their properties and role in the formation of massive galaxies are
summarized.
Radio galaxies form a subclass of active galactic nuclei (AGN). Other AGN are the
optically luminous quasi stellar objects (QSOs), Seyferts 1 and 2, and Blazars. The
current understanding is that AGN are powered by massive black holes (106−9 M )
that accrete matter through an accretion disk. Because the conversion into energy of
matter that falls into a black hole can happen very efficiently (∼ 10% of the rest mass
can be radiated away before it crosses the Schwarzschild radius; more in spinning
black holes), this results in prodigious amounts of radiation being emitted from a small
central region. Hence the name “active galactic nuclei”. In certain cases (e.g., QSOs)
these small inner regions can outshine the approximately 100 billion stars that make up
a typical galaxy. Consequently, the term AGN is used somewhat loosely to designate
both the central regions and the galaxies hosting AGN.
The defining characteristic of radio galaxies is that they emit strongly at radio wavelengths (P178 MHz > 1026 WHz−1 ). This is over two orders of magnitude more than radioquiet AGN, and there seems to a be dichotomy between the two classes. The powerlaw spectrum and high polarization indicate that the process responsible for the radio
emission is synchrotron radiation. Although the exact mechanism producing the synchrotron radiation in radio galaxies is not fully understood, it likely involves efficient
accretion of matter onto a spinning super massive (∼ 109 M ) black hole (Rees 1978;
Blandford & Payne 1982).
Not only the mechanism responsible for producing powerful radio structures, but
also the evolution of these structures is subject to debate. The typical life time of radiosource activity is short, 10–100 Myr. The observed range in sizes of z ∼ 0.5 powerful radio sources, from subgalactic (< 1 kpc) to cluster scales (> 1 Mpc) has been interpreted as evidence for evolution of radio source size with age. Presumably, radio
sources begin in the very compact Gigahertz Peaked Spectrum (GPS; < 1 kpc) phase,
pass through the Compact Steep Spectrum (CSS; 1–20 kpc) stage, and ultimately evolve
into the classical edge brightened Fanaroff & Riley class II (FR II; Fanaroff & Riley 1974)
radio sources.
Historically, interest in AGN has been their use as cosmological probes. Because
they are extremely luminous they previously were the only objects that could be seen
out to large distances. With the availability of 10-m class telescopes thousands of “normal” galaxies at redshifts z > 2 have been discovered (Steidel et al. 1996; Steidel et al.
1999; Labbé et al. 2003). AGN are extreme sources and very rare in the local universe.
It has therefore been argued that they are interesting, but statistically not significant.
However, there are at least two reasons why radio galaxies are not “freaks” but instead
signify an important phase in the evolution of massive galaxies. First, the discovery of
the tight correlation, mentioned earlier, between black hole mass and stellar mass indicates the importance of (feedback from) massive black holes. Secondly, the observation
4
Chapter 1. Introduction
that the space density of AGN was much larger in the past (the population peaks at
redshifts of z =2–3) and behaves in a similar way as the estimated star formation rate
density, suggests that AGN and star formation are closely linked. It is therefore well
possible that every massive galaxy once was a powerful radio source, albeit for a short
period.
At low redshifts (z < 1), powerful radio sources are uniquely identified with massive elliptical galaxies. There is a tight correlation between the near-infrared K-band
magnitude of radio galaxies and their redshift (the “Hubble” K − z relation; De Breuck
et al. 2002). It seems to trace the most massive systems at any epoch and suggests
that radio galaxies are tracers of possible over-dense regions and the sites of forming
massive galaxies. In scenarios of biased galaxy formation these would mark the peaks
in the early density field, and are expected to be at the centers of forming clusters of
galaxies. Studies of high redshift (z > 2) clusters of galaxies are important to constrain
galaxy evolution and cosmological models (Bahcall & Fan 1998), and the radio galaxies themselves are important for constraining the upper mass end of models of galaxy
formation, where current models fail.
Although both radio galaxies and QSOs host massive black holes, emit energy over
large wavelength ranges, and can be found at high redshift, radio galaxies are preferred for studies of galaxy formation. This is because QSOs outshine their host galaxies completely, whereas for radio galaxies the bright central region is thought to be
blocked by an optically thick torus. Therefore, in radio galaxies the extended host
galaxy can be studied in detail. Because star formation mostly occurs in dust obscured
regions, optical surveys for star formation give a biased view and large correction factors are required. The long wavelength selection of radio galaxies is not sensitive to
such obscuration and can therefore help to constrain the relative fractions of unobscured and obscured star formation. Furthermore, radio galaxies are often embedded
in large > 100 kpc gaseous halos (Chapter 6, 7; van Ojik et al. 1996; Villar-Martı́n et al.
2003) from which they could be forming. Because of their large spatial extents these
gaseous reservoirs are excellent laboratories for studying feedback processes at high
redshift in detail.
Consequently, many searches for distant radio galaxies (z > 2) have been conducted
(e.g., Röttgering et al. 1997; De Breuck et al. 2001). Because radio galaxies are rare, large
numbers of foreground galaxies have to be filtered out to find high redshift candidates.
The most efficient selection technique is based on spectroscopic follow-up of sources
1400MHz <
α
with ultra steep radio spectra (USS; α325MHz
∼ −1.30; Sν ∝ ν ), which is analogous to
a “red radio color” (e.g., Blumenthal & Miley 1979). De Breuck et al. (2000) constructed
a large sample of 669 ultra steep spectrum radio sources for this specific purpose. The
galaxies were selected from a radio sample with flux densities 10 < S 1.4GHz < 100 mJy,
fainter than most previous surveys. This recipe for finding distant sources has proved
highly successful with spectroscopic followup of these sources with faint near-infrared
counterparts showing emission line based redshifts z > 3 in ∼35% of the cases (for
details see De Breuck et al. 2001). Presently, of order 150 radio galaxies are known at
redshifts z > 2. They constitute an unique sample to study the formation of massive
galaxies and the role of feedback processes therein.
Section 3. Outline of this thesis
5
3 Outline of this thesis
This thesis aims to study the evolution of massive galaxies by focusing on three interrelated ingredients of radio galaxies: gas, dust, and star formation. The work is based
on a large amount of observations with ground- and space-based telescopes, from the
radio through X-ray wavelength range.
Gas
Spectroscopic observations of distant radio galaxies have shown that they are embedded in large gaseous halos. The role of these halos in galaxy formation is not yet
fully understood. Answers to the following questions could provide a better understanding: What is the extent of the halos? Which processes provide the energy that
ionizes these halos? Is the gas infalling in a cooling flow or is the gas the result of
outflows and radiation from starburst and AGN? How does this affect the galaxy formation process? Are active, massive forming galaxies capable of enriching the intracluster media with metals and thus affect cluster evolution? Only few high-quality
images of these halos existed prior to the work in this thesis. Higher quality images
are of great interest because of the potential diagnostics they may provide about the
very early stages of galaxy formation, and about starburst/AGN feedback and chemical enrichment during this process. Chapter 6 presents the deepest of such images ever
obtained. Follow-up optical and near-infrared spectroscopy is presented in Chapter 7.
Star formation
In the case of the radio galaxy 4C 41.17 there is direct evidence for massive star formation (up to ∼ 1500 M yr−1 after correction for extinction) based on stellar absorptionlines (Dey et al. 1997). Recent mm-interferometry studies of CO line and continuum
emission for three z > 3 distant radio galaxies have shown that the star formation occurs galaxy wide over distances up to 30 kpc (Papadopoulos et al. 2000; De Breuck et al.
2003). Together this suggests that we are observing not merely scaled up versions of
local ultraluminous infrared galaxies (ULIRGs) where the bursts are confined to the inner few kpc, but wide-spread starbursts within which the galaxies are forming the bulk
of their eventual stellar populations. Chapter 5 presents a theoretical investigation of
the physical parameters controlling the spectral energy distributions of such extended
starbursts.
Dust
Dust is expected to play an important role in star forming regions. It absorbs
UV/optical radiation from the starburst and reradiates it at far-infrared wavelengths.
Optical searches for distant galaxies (e.g., using the Lyman-break technique; Steidel et
al. 1996; Steidel et al. 1999) are thus likely to be biased against dusty objects. Finding
distant star forming galaxies through submillimeter (rest-frame FIR) emission selects
only the most obscured sources. So far there has been little overlap between the opti-
Chapter 1. Introduction
6
cal and submillimeter selected star forming sources. It remains unclear whether they
are members of a continuous population (e.g., Adelberger & Steidel 2000; Webb et al.
2003) and arguments have been made that either one of them could dominate the star
formation density at high redshift. Since selection at radio wavelengths circumvents
the aforementioned selection biases it could help determine the relative contributions
of obscured and unobscured star formation to the star formation history of the universe. Until recently, many searches for dust in distant radio galaxies proved largely
unsuccessful. Chapter 2 is a study of the change of dust obscured star formation rate
in radio galaxies over a large range in redshift. Chapters 3 and 4 investigate the role of
dust in young distant radio sources.
A brief outline of each chapter is given below.
Chapter 2
Multi-wavelength observations of distant radio galaxies have provided considerable
information about how the formation and evolution of present day brightest cluster
galaxies must have taken place. One of the great uncertainties is the role of dust. Dust
is produced by starbursts. This dust absorbs most of the ultraviolet/optical light from
the young hot stars in the starbursts and re-emits it in the infrared waveband. Therefore far-infrared emission is often used as a measure of the star formation rate. Emission for cool (∼ 30–50 K) dust has a spectrum with maximum intensity at far-infrared
(∼ 60–100 µm) wavelengths. For very distant (z > 1) galaxies this peak is redshifted
to submillimeter (200–1000 µm) wavelengths. At observed wavelengths of 850 µm the
dimming of galaxies due to increasing distance is compensated for as the far-infrared
peak shifts into the bandpass. Instruments that observe this wavelength are (almost)
equally sensitive to star formation in galaxies up to redshifts of z ∼ 10, as they are to
galaxies at z ∼ 1 and are well-suited for studying star formation over large cosmological timescales.
We therefore initiated an observing program with the Submillimetre Common User
Bolometer Array (SCUBA) to measure the dust continuum emission from 24 z > 1 radio galaxies. We detected submillimeter emission in 12 galaxies, including 9 detections
at z > 3. When added to previous published results the data almost triple the number of radio galaxies with z > 3 detected in the submillimeter and yield a sample of
69 observed radio galaxies over the redshift range z = 1–5. We find that the galaxies are luminous at submillimeter wavelengths. We confirm and strengthen the result
from previous submillimeter observations of radio galaxies that the detection rate is a
strong function of redshift. We compare the redshift dependence of the submillimeter
properties of radio galaxies with those of quasars and find that for both classes of objects the observed submillimeter flux density increases with redshift to z ≈ 4, beyond
which, for the galaxies, we find tentative evidence for a decline.
If this change in submillimeter flux is due to a change in the intrinsic star formation
rate, it is consistent with a scenario in which the bulk of the stellar population of radio
galaxies forms rapidly around redshifts of z = 3 − 5 after which they are more passively evolving (c.f. Best, Longair, & Röttgering 1998). However, dust can be heated
Section 3. Outline of this thesis
7
by UV/optical radiation not only from starbursts but also from AGN. This complicates
the use of submillimeter fluxes as a measure of star formation rate. We have therefore
searched for evidence of possible contamination by AGN. The lack of evidence for a
correlation between radio-power and submillimeter emission and an anti-correlation
between submillimeter luminosity and fractional polarization of the UV continuum
(AGN light is expected to be highly polarized) indicate that starbursts are the dominant source of heating for dust in radio galaxies. We conclude that distant radio galaxies are massive forming galaxies, forming stars at rates up to a few thousand M yr−1 .
Chapters 3 and 4
1400MHz <
α
As described above, ultra steep radio spectrum (USS; α 325MHz
∼ −1.30; Sν ∝ ν ) selection is an efficient criterion for finding distant radio galaxies. However, a large fraction
(∼ 30%) of selected high redshift radio galaxy candidates fails to show emission lines in
deep spectroscopic exposures at Keck, even though some galaxies are detected in the
< 2 00 ) radio morpholocontinuum. Generally, they are characterized by compact (θ ∼
gies. The compact radio structures could indicate that we observe them shortly after
the onset of the radio activity and that they are possibly very young. The observation
that their parent objects are only detected in the near-infrared suggests that they are
heavily obscured and/or at very high redshift. This is of interest because a population of high redshift heavily obscured AGN seems the best candidate to account for
a substantial fraction (30−40%) of the 5−10 keV cosmic X-ray background (XRB). In
both scenarios their host galaxies could be expected to be forming stars at large rates.
Therefore, these sources are of interest for studying radio source evolution and particularly for studying possible connections between star formation and the onset of radio
source activity. To search for signatures of dust and help constrain the nature and redshifts of these “no-z” radio galaxies, we have conducted a program of submillimeter
and millimeter observations.
In Chapter 3, we report the results of a detailed study of one of these objects,
WN J0305+3525. It appears associated with a small group of faint near-infrared (K ∼
21–22) objects and it is a strong detection at both 850 µm and 1.25 mm. On the basis of its faint K-band magnitude, spectral energy distribution and other evidence we
estimate that the radio galaxy is probably at a redshift z ' 3 ± 1. This would make
WN J0305+3525 a radio-loud hyper luminous infrared galaxy (L FIR ∼ 1013 L ) similar
to, but more obscured than, other dusty radio galaxies in this redshift range.
Chapter 4 describes the results of a submillimeter survey for ten such “no-z” compact USS sources (including WN J0305+3525) and includes Chandra X-ray observations for the three strongest submillimeter detections. In total four galaxies were detected in the submillimeter with S/N > 4. This submillimeter detection fraction is
close to the one for z > 2.5 radio galaxies in surveys of comparable sensitivity (Chapter 2; Archibald et al. 2001). Also, a relatively strong statistical signal h S CSS
850,<3σ i =
2.48 ± 0.48 mJy was found in the stacked non-detections. Together, this indicates that
an obscured phase and the triggering of radio sources are connected, and that this
compact obscured phase may be a common ingredient in AGN evolution.
None of the galaxies was detected with Chandra. Assuming redshifts z ∼ 3 and
8
Chapter 1. Introduction
based on the empirical radio to X-ray relation for AGN, intrinsic X-ray luminosities
of L2−10 keV ∼ 1044−46 erg s−1 and column densities of a few 1023 cm−2 are inferred. This
shows that if many AGN go through such a compact obscured stage, they could maybe
contribute to the hard X-ray background.
Chapter 5
As mentioned above, both AGN and starbursts can heat dust in galaxies. Many previous studies have reported that the fraction of far infrared luminosity ( L FIR ) that is
contributed by an AGN increases with LFIR . In order to infer reliable star formation
rates, quantification of the respective contributions is essential. We have therefore examined the far-infrared spectral energy distributions (SEDs) of 41 local ultra luminous
infrared galaxies (ULIRGs) using data from the literature. These observed SEDs are
fitted with SEDs that were constructed by coadding the output from ionization models for dynamically evolving H II regions with a sophisticated treatment of embedded
dust. The theoretical SEDs include a contribution from a dusty narrow line region in
order to model enshrouded AGN.
From these models it is found that almost all ULIRGs are best fitted with high
(P/k > 106 cm−3 K) pressures of the interstellar medium, similar to the pressures inferred from emission line ratios. These high pressures are likely related to the pressure
above which blowout occurs in a galactic superwind.
We have investigated the implications of our findings for high redshift sources. It
seems that the physical mechanisms controlling their SEDs may be similar to those for
local ULIRGs, and that the models are applicable also in the distant universe. Fitting
of three high redshift radio galaxies with good far-infrared observations indicates that
to fit their SEDs may require even higher pressures than found for ULIRGs. Such high
pressures could be related to the presence of the radio source and possibly jet induced
star formation. They would increase the effective temperature of these highly luminous sources and agree with the recently reported far infrared luminosity-temperature
relation (Blain et al. 2004).
We find that the relative contribution of an embedded AGN can vary significantly.
Taking this AGN related component into account can decrease star formation rates as
inferred from LFIR by factors of 2–3. For the high redshift radio galaxies, the high dust
temperatures together with hidden AGN activity would decrease the star formation
rates to values lower than is commonly inferred.
Chapter 6
In the previous chapters we focused on the star formation rates in connection with radio source evolution. Here we discuss the gaseous environments of the radio galaxies.
Lyα nebulae may be the first evidence for accretion in large dark matter halos. They
may signal the formation of massive galaxies through merging of smaller starburst
systems or cooling flows.
We report deep Keck narrow-band Lyα images of the luminous z > 3 radio galaxies
4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100–200 kpc scale emis-
Section 3. Outline of this thesis
9
sion line nebulae, centered on these galaxies, which exhibit a wealth of morphological
structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone–shaped structures and (indirect) evidence for extended
Lyα absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large CDM halos, forming stars and multiple stellar
systems. Merging of these “building blocks” triggers large scale starbursts, forming
the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes
which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation may disrupt the accretion process, limiting galaxy and black
hole growth, and imprint the observed filamentary and cone-shaped structures of the
Lyα nebulae.
Chapter 7
In this final chapter, we present deep optical and near-infrared spectroscopic data of
the giant nebular emission line halos described in Chapter 6. Previous optical studies
found that the inner high surface-brightness regions exhibit disturbed kinematics with
velocity dispersions >1000 km s−1 that seem to be closely related to the radio source.
The outer regions of the halos exhibit kinematics with typical velocity dispersions of a
few hundred km s−1 , and velocity characteristics consistent with rotation.
To investigate this further we obtained additional optical spectroscopic data at position angles perpendicular to the radio axes. Since Lyα is subject to resonance scattering, interpretation of the inferred kinematics is difficult. We therefore performed nearinfrared spectroscopy for these emission line halos, targeting [O II] and [O III]. Evidence for the presence of enriched material (oxygen) throughout the nebula of 4C 41.17
(up to a distance of ∼60 kpc along the radio-axis) is found. The oxygen emission has
a similar spatial and kinematic distribution as the Lyα emission. We argue that this
implies that the Lyα cannot be purely scattered light, and that the halo had already
been enriched by a previous generation of stars. It is possible that the extended oxygen has been transported from the central starburst region aided by the radio source.
We discuss various feedback processes and their implications for galaxy formation in
the context of the nature and origin of these halos. The evidence presented here could
help cast observational light on the good correlation found in galaxies between the stellar velocity dispersion and the black hole mass. Silk & Rees (1998) and Sazonov et al.
(2004) have speculated that this results from outflows which are driven by radiation
pressure provided by the black hole. In the case of 4C 41.17 we have good evidence
that the radio lobes are driving this outflow, assisted by radiation pressure and starburst winds. Together these data help us to better understand the formation of cluster
ellipticals. It seems that the radio sources, starburst superwinds and AGN radiation
may disrupt the accretion process limiting galaxy and black hole growth, and possibly
help enrich the inter cluster medium. These processes may help shape the high-mass
end of the galaxy luminosity function.
10
Chapter 1. Introduction
4 Future prospects
The cosmological parameters have recently been constrained with unprecedented precision and the basic concepts of galaxy formation and evolution seem to be understood.
This leads to a situation in which we can hope to start answering outstanding questions
in the formation of massive galaxies with some confidence.
Using the data presented in this thesis we have shown that the evolution of radioloud AGN couples strongly with the evolution of their host galaxies. However, many
more observations and better modeling are required to investigate this in a quantitative
way and to obtain a continuous sampling from the present to earlier cosmic epochs.
The giant emission line nebulae around radio galaxies provide important laboratories for studies of feedback from central regions of radio galaxies. Deep spectroscopy
targeting non-resonant emission lines are important to constrain various feedback scenarios, and determine the gas kinematics, metalicities and sources of ionization. To
fully exploit narrow-band emission-line images as a tool for studies of galaxy formation requires the use of optical and near-infrared integral field units (2-D spatial + 1-D
spectral, imaging devices) on 8-10 m (and 30-100 m?) telescopes with advanced adaptive optics systems (e.g., van Breugel & Bland-Hawthorn 2000).
We are now at a time where unprecedented quality observations are driving models
forward. The high quality data will allow better constraints on the physical processes
that are important for galaxy formation. Models will be able to move away from semianalytical approaches and include more of the real physics. Models for galaxy evolution depend heavily on the full coverage of the spectral energy distributions. Spitzer
and the future ALMA and Herschel will be crucial for this in the mid-infrared to millimeter regime and therefore for studies of the star formation rates over large cosmological times. Furthermore, these instruments will provide means of estimating the
masses of distant galaxies through observing the dynamics of molecular line emission
and observations of their stellar populations in the rest-frame near-infrared.
Cosmology is an observationally driven science. The greatest leaps forward came
with the arrival of new instruments opening up new wavelength regimes or providing data of order of magnitude better sensitivity and resolution. In the coming years
it is almost guaranteed that instruments such as LOFAR and XEUS will once again
revolutionize our view of the universe. XEUS will provide information on the first
massive black holes at redshifts up to z ∼ 10. LOFAR will provide extreme sensitivity in presently poorly explored frequency regimes. It is likely to discover many new
exciting astrophysical objects. Further, LOFAR is expected to detect radio galaxies at
redshifts z ∼ 8. This will allow us to study the coevolution of massive galaxies and
their central black holes to even earlier cosmic epochs than is presently possible.
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Chapter 2
Dust and star formation in distant radio
galaxies
Michiel Reuland, Huub Röttgering, Wil van Breugel, and Carlos De Breuck, Monthly
Notices of the Royal Astronomical Society, Vol. 353, p. 377, 2004
We present the results of an observing program with the SCUBA bolometer array to measure the submillimetre (submm) dust continuum emission of 24 distant
(z > 1) radio galaxies. We detected submm emission in 12 galaxies with S/N > 3,
including 9 detections at z > 3. When added to previous published results these
data almost triple the number of radio galaxies with z > 3 detected in the submm
and yield a sample of 69 observed radio galaxies over the redshift range z = 1–5.
We find that the range in rest-frame far-infrared luminosities is about a factor of
10. We have investigated the origin of this dispersion, correlating the luminosities
with radio source power, size, spectral index, K-band magnitude and Lyα luminosity. No strong correlations are apparent in the combined data set. We confirm and
strengthen the result from previous submm observations of radio galaxies that the
detection rate is a strong function of redshift. We compare the redshift dependence
of the submm properties of radio galaxies with those of quasars and find that for
both classes of objects the observed submm flux density increases with redshift to
z ≈ 4, beyond which, for the galaxies, we find tentative evidence for a decline. We
find evidence for an anti-correlation between submm luminosity and UV polarisation fraction, for a subsample of 13 radio galaxies, indicating that starbursts are the
dominant source of heating for dust in radio galaxies.
1 Introduction
T
HERE is strong evidence that powerful high redshift radio galaxies (HzRGs; z >
2) are the progenitors of the brightest cluster ellipticals seen today. HzRGs are
the infrared brightest and presumably the most massive galaxies at any epoch (De
Breuck et al. 2002) and host actively-accreting super massive black holes with masses
of order 109 M (Lacy et al. 2001; Dunlop et al. 2003). Therefore, they are key objects
for studying the formation and evolution of massive galaxies and super-massive black
holes.
HzRGs are likely to be in an important phase of their formation process for several
reasons: They have large reservoirs of gas from which they could be forming, as shown
by spectacular (> 100 kpc) luminous Lyα haloes (e.g., McCarthy 1993; van Ojik et al.
13
14
Chapter 2. Dust and star formation in distant radio galaxies
1996; Reuland et al. 2003) and widespread H I absorption features in the Lyα profiles
(van Ojik et al. 1997). Their rest-frame UV morphologies are characterized by clumpy
structures, similar to the Lyman-break galaxies at z ∼ 3 , that will merge with the central galaxy on dynamical time-scales of 108 yrs (Pentericci et al. 1998; Pentericci et al.
1999). In the case of 4C 41.17 there is direct evidence for massive star formation (up to
∼ 1500 M yr−1 after correction for extinction) based on stellar absorption–lines (Dey
et al. 1997). Finally, mm-interferometry studies of CO line and continuum emission
for three z > 3 HzRGs have shown that the star formation occurs galaxy wide over
distances up to 30 kpc (Papadopoulos et al. 2000; De Breuck et al. 2003) and there is
even evidence for star formation on scales of 250 kpc (Stevens et al. 2003). Together
this suggests that we are observing not merely scaled up versions of local ultraluminous infrared galaxies (ULIRGs) where the bursts are confined to the inner few kpc,
but wide-spread starbursts within which the galaxies are forming the bulk of their
eventual stellar populations.
HzRGs are an important sample for studying the star formation history of the universe because their selection is based on long wavelength radio emission whose propagation is not affected by the presence of dust. Dust is expected to play a significant role
in star forming regions, absorbing UV/optical radiation from the starburst and reradiating it at far-infrared (FIR) wavelengths (Sanders & Mirabel 1996). Optical searches
for distant galaxies (e.g., using the Lyman-break technique; Steidel et al. 1996; Steidel
et al. 1999; Ouchi et al. 2001) are thus likely to be biased against dusty objects. Finding
distant star forming galaxies through submillimetre (submm; rest-frame FIR) emission
(e.g., Hughes et al. 1998; Bertoldi et al. 2002; Chapman et al. 2002a; Cowie et al. 2002;
Scott et al. 2002; Smail et al. 2002; Webb et al. 2003a; Eales et al. 2003) selects only the
most obscured sources. So far there has been little overlap between the optical and
submm selected star forming sources (selection on very red near-IR colours may prove
more fruitful; e.g., Frayer et al. 2004). It remains unclear whether they are members
of a continuous population (e.g., Adelberger & Steidel 2000; Webb et al. 2003b) and
arguments have been made that either one of them could dominate the star formation
density at high redshift (Blain et al. 1999; Adelberger & Steidel 2000). Since selection at
radio wavelengths circumvents the aforementioned selection biases it could help determine the relative contributions of obscured and unobscured star formation to the
star formation history of the universe.
Archibald et al. (2001,hereafter A01) have conducted the first systematic submm
survey to study the star formation history of radio galaxies over a redshift interval of
0.7 < z < 4.4. In their sample of 47 galaxies, they found evidence for a considerable
range in FIR luminosities, a substantial increase in 850 µm detection rate with redshift
and that the average 850 µm luminosity rises at a rate (1 + z)3−4 out to z ' 4. These
results prompt the following questions: Is the dispersion in FIR luminosities due to
differences in their star formation rates or dust contents? Does the strong increase with
redshift reflect an increase in star formation rates or could it be related to changing
dust properties? Does the FIR luminosity keep on rising with redshift or does it level
off and is there a redshift cut-off? Are the inferred star formation rates comparable to
those derived from the optical/UV? Do the submm properties of quasars (QSOs) and
radio galaxies show similar trends or do the two classes of objects evolve differently?
Section 1. Introduction
Source
WNJ0528+6549
MRC1138−262
WNJ1115+5016
WNJ0747+3654
WNJ0231+3600
B3J2330+3927
TNJ1112−2948
MRC0316−257
PKS1354−17
WNJ0617+5012
MRC0251−273
WNJ1123+3141
WNH1702+6042
TNJ0205+2242
TNJ0121+1320
6C1908+722
WNJ1911+6342
MG2141+192
WNJ0346+3039
4C60.07
TNJ2007−1316
TNJ1338−1942
TNJ1123−2154
TNJ0924−2201
z
1.210
2.156
2.550
2.992
3.079
3.086
3.090
3.130
3.150
3.153
3.160
3.217
3.223
3.506
3.516
3.532
3.590
3.592
3.720
3.791
3.830
4.100
4.109
5.190
15
h
5
11
11
7
2
23
11
3
13
6
2
11
17
2
1
19
19
21
3
5
20
13
11
9
RA(J2000)
m
s
28
46.07
40
48.25
15
6.87
47
29.38
31
11.48
30
24.91
12
23.86
18
12.06
47
96.03
17
39.37
53
16.70
23
55.85
3
36.23
5
10.69
21
42.74
8
23.70
11
49.54
44
7.50
46
42.68
12
55.15
7
53.23
38
26.06
23
10.15
24
19.92
DEC (J2000)
LAS
◦
0
00
00
+ 65
− 26
+ 50
+ 36
+ 36
+ 39
− 29
− 25
− 17
+ 50
− 27
+ 31
+ 60
+ 22
+ 13
+ 72
+ 63
+ 19
+ 30
+ 60
− 13
− 19
− 21
− 22
49
29
16
54
0
27
48
35
44
12
9
41
38
42
20
20
42
29
39
30
16
42
54
1
57.3
10.1
23.9
38.1
26.6
11.2
6.2
9.7
02.2
54.7
9.6
26.1
52.2
50.3
58.3
11.8
9.6
15.0
49.3
51.0
43.6
30.1
5.3
41.5
1.9
15.8
0.2
2.1
14.8
1.9
9.1
7.6
−
3.4
3.9
25.8
11.5
2.7
0.3
14.4
1.8
8.5
0.4
16.0
7.2
5.5
0.8
1.2
FLyα
cgs
−
13.9
2.0
0.8
1.1
4.4
2.9
2.4
−
0.8
−
6.2
−
−
−
32.0
1.4
6.2
−
10.1
2.5
10.1
0.2
0.4
K
mag
18.2
16.1
19.2
20.0
−
18.8
−
−
−
19.7
−
17.5
−
18.8
18.8
16.5
18.6
19.3
17.8
19.3
17.9
19.7
20.4
19.9
References
DB00a, dV03
Röt97, Pen97, DB00b
DB00a, DB01, DB02
DB00a, DB01, DB02
DB00a, DB01, DB02
DB03a
DB00a, DB01
McC90, ER96, DB00b
Dri97
DB00b, DB01, DB02
McC96, Kap98
DB00a, DB01, DB02
Ren98
DB00a, DB01, DB02
DB00a, DB01, DB02
Dey99, Pap00, DB01
DB00a, DB01, DB02
S99
DB00a, DB02, dV03
Röt97, Pap00, DB00b
DB00a, DB02, DB03b
DB99
DB00a, DB01, DB02
vB99
Table 1 — Redshifts, radio positions, largest angular sizes, Lyα fluxes, K-band magnitudes and references to papers from which these data were taken for all objects that were observed in our submm program. The Lyα fluxes are in units of 10−16 erg s−1 cm−2 , and the K-band magnitudes were measured in
a 64 kpc diameter aperture where possible. References: DB99,DB00a,DB00b,DB01,DB02,DB03a,DB03b=
De Breuck et al. (1999, 2000); De Breuck et al. (2000); De Breuck et al. (2001); De Breuck et al. (2002,
2003,De Breuck et al. in preparation), dV03 = de Vries et al. in preparation, Dey99 = Dey (1999), Dri97 =
Drinkwater et al. (1997), ER96 = Eales & Rawlings (1996), McC90 = McCarthy et al. (1990), McC96 = McCarthy et al. (1996), Kap98 = Kapahi et al. (1998), Pap00 = Papadopoulos et al. (2000), Pen97 = Pentericci
et al. (1997), Ren98 = Rengelink (1998), Röt97 = Röttgering et al. (1997), S99 = Stern et al. (1999), vB99 =
van Breugel et al. (1999).
The submm findings from A01 were based on a limited number of detections at
high redshift (z > 3). To put these results on a statistically firmer footing and search
for possible correlations with other galaxy parameters more submm observations were
required. Here we present such observations of all z > 3 HzRGs known at the beginning of 2001 (e.g., De Breuck et al. 2001) which had not been observed in the submm.
Adding these to the survey of A01 almost triples the number of detections at high redshift, creating a sample which is statistically significant over the full redshift range z =
1 – 5.
The structure of this paper is a follows: the sample selection, observations and
data analysis are described in Section 2. Results and notes on some individual sources
are presented in Section 3. Various correlations with submm properties of HzRGs are
investigated in Section 4 and described in detail in Section 5. Section 6 presents a
comparison between HzRGs and QSOs. We discuss and summarize our conclusions
in Section 7. Throughout this paper, we adopt a flat universe with ΩM = 0.3, ΩΛ = 0.7,
and H0 = 65 km s−1 Mpc−1 . Using this cosmology the look-back time at z ∼ 2.5 (the
1
median redshift of our sample) is 11.7 h−
65 Gyr and a galaxy at such a redshift must be
1
less than 2.8 h−
65 Gyr old.
Chapter 2. Dust and star formation in distant radio galaxies
16
Source
WNJ0528+6549
MRC1138−262
WNJ1115+5016
WNJ0747+3654
WNJ0231+3600
B3J2330+3927
TNJ1112−2948
MRC0316−257
PKS1354−17
WNJ0617+5012
MRC0251−273
WNJ1123+3141
WNH1702+6042
TNJ0205+2242
TNJ0121+1320
6CJ1908+722
WNJ1911+6342
MG2141+192
WNJ0346+3039
4C60.07
TNJ2007−1316
TNJ1338−1942
TNJ1123−2154
TNJ0924−2201
z
Nint
× 50
1.210
2.156
2.550
2.990
3.080
3.086
3.090
3.130
3.150
3.153
3.160
3.220
3.223
3.506
3.517
3.532
3.590
3.592
3.720
3.791
3.830
4.100
4.109
5.190
4+4
2
4
6
7
3
5
2
2
6+6
2
8
1
6
7+4
6
2
7+5
4
5
5
4+7
2
8+4
S850 a
mJy
−1.9 ± 1.3
12.8 ± 3.3b
3.0 ± 1.3
4.8 ± 1.1
5.9 ± 1.6
14.1 ± 1.7c
5.8 ± 1.1
0.6 ± 2.7
20.5 ± 2.6d
1.0 ± 0.7
0.6 ± 2.8
4.9 ± 1.2
−0.4 ± 3.6
1.3 ± 1.3
7.5 ± 1.0
10.8 ± 1.2c
1.3 ± 3.6
2.3 ± 0.9b,e
−0.5 ± 1.3
11.5 ± 1.5b,c,e
5.8 ± 1.5
6.9 ± 1.1
1.5 ± 1.7
−0.7 ± 1.1
S/N
Quality
3σ lim.
mJy
−1.4
3.9
2.3
4.5
3.7
8.5
5.1
0.2
8.0
1.3
0.2
4.1
−0.1
1.0
7.6
9.0
0.4
2.6
−0.4
7.6
4.0
6.2
0.9
−0.7
A
B
A
A
B
A
A
B
B
B
A
A
B
A
A
A
B
A
A
A
A
A
A
A
<3.9
<6.9
<8.8
<3.2
<8.9
<10.8
<5.2
<11.9
<5.0
<3.8
<6.7
<3.2
S450
mJy
2 ± 29
-65 ± 134
-20 ± 11
18 ± 15
-29 ± 22
49 ± 18
15 ± 9
5 ± 48
-47 ± 77
3 ± 16
-54 ± 91
4 ± 14
-73 ± 91
27 ± 23
4 ± 16
33 ± 17
-38 ± 50
12 ± 13
-5 ± 12
10 ± 13
4 ± 45
-36 ± 32
-7 ± 11
-0 ± 26
L850
W Hz−1 sr−1
<23.05
23.26b
<23.31
23.15
23.23
23.61
23.23
<23.41
23.77
<22.96
<23.41
23.15
<23.49
<23.17
23.33
23.49
<23.53
22.96b
<23.02
23.61b
23.21
23.29
<23.27
<22.94
LFIR
L
<12.63
12.83b
<12.90
12.73
12.81
13.19
12.81
<12.99
−
<12.55
<12.99
12.73
<13.07
<12.75
12.91
13.07
<13.11
<12.55b
<12.61
13.19b
12.79
12.87
<12.85
<12.53
L3GHz
W Hz−1 sr−1
24.59
27.15
25.95
26.22
26.27
26.56
26.66
27.22
27.71
26.10
27.00
26.60
26.40
26.58
26.55
27.25
26.26
27.30
26.43
27.15
26.98
27.05
26.76
27.24
Table 2 — Observed 850 µm and 450 µm submm flux densities S 850 µm and S450 µm with their standard
errors for the radio sources in the program. The total duration of the observations, Nint is given in sets
of 50 integrations. 3σ upper limits to the 850 µm flux are shown for sources whose S/N is below 3.
Only B3 J2330+3927 may have been detected at 2σ at 450 µm. Logarithms of inferred rest-frame 850 µm
luminosities L850 , far-IR luminosities, LFIR and radio luminosities L3GHz are shown for the dust template
with β = 1.5, Td = 40 K and a flat universe with ΩM = 0.3, ΩΛ = 0.7, and H0 = 65 km s−1 Mpc−1 .
a
This does not include the 10–15 per cent uncertainty in absolute photometric calibration.
b
For the statistical analysis we use S850 = 5.9 ± 1.1 mJy, S850 = 3.3 ± 0.7 mJy and S850 = 14.4 ± 1.0 mJy
for MRC 1138−262, MG 2141+192 and 4C 60.07, respectively. L 850 and LFIR were inferred using those
values. See Section 4 for details.
c
Data published in CO imaging studies by Papadopoulos et al. (2000) and De Breuck et al. (2003).
d
PKS 1354−17 is likely to be dominated by non-thermal emission (c.f. Section 2.2).
e
Also part of the survey by A01.
2 Sample Selection and Observations
The observations presented here include submm observations of distant radio galaxies.
The targets were selected from an increasing sample of HzRGs that is the result of an
ongoing effort by our group and others (De Breuck et al. 2000, 2001,de Vries et al.
in preparation; Spinrad private communication) to find distant radio galaxies based
on Ultra Steep Radio Spectrum (USS; i.e. red radio color) and near-IR identification
selection criteria (for details see De Breuck et al. 2001).
> 3 and decWe selected all HzRGs known at the beginning of 2001 with redshifts z ∼
◦
lination δ > −30 that did not have prior submm observations. Our aim was to observe
a significant sample of HzRGs to complement the observations of A01 and, in particular, to obtain better statistics at the highest redshifts. MG 2141+192 and 4C 60.07 were
observed in both programs, because, at the time of observation, their inclusion in the
A01 sample was unknown to us. The 850 µm results for B3 J2330+3927, 6C J1908+722,
Section 2. Sample Selection and Observations
17
and 4C 60.07 have been published previously as part of their CO imaging studies (Papadopoulos et al. 2000; De Breuck et al. 2003). MRC 1138−262 was included in the
program because of its wealth of supporting data (e.g., Pentericci et al. 1997; Carilli
et al. 2002) and WN J1115+5016 because it is one of only two radio galaxies showing a
broad absorbtion line (BAL) system (De Breuck et al. 2001), the other BAL radio galaxy,
6C J1908+722, being a strong CO emitter (Papadopoulos et al. 2000). WN J0528+6549 at
z = 1.210 was observed because it was first thought to be at redshift z = 3.120 (actually
belonging to another galaxy on the slit).
The coordinates, redshifts, largest angular sizes of the radio sources, Lyα fluxes, Kband magnitudes, and references for the full sample observed in the submm are listed
in Table 1.
2.1 SCUBA photometry
The observations were carried out between October 1997 and January 2002 with the
Submillimetre Common–User Bolometer Array (SCUBA; Holland et al. 1999) at the
15 m James Clerk Maxwell Telescope (JCMT). We observed at 450 µm and 850 µm
wavelengths resulting in beam sizes of 7.5 00 and 14.7 00 respectively. We employed the
9-point jiggle photometry mode, which samples a 3 × 3 grid with 2 00 spacing between
grid points, while chopping 45 00 in azimuth at 7.8 Hz. Frequent pointing checks were
performed to ensure pointings better than 2 00 and reach optimal sensitivity.
Our original goal was to observe all sources down to 1 mJy rms at 850 µm. This
is a sensible limit, since at 850 µm confusion becomes a problem for sources weaker
than 2 mJy (Hughes et al. 1998; Hogg 2001), it is obtainable in 3 hrs per source, and
it is matched to the survey by A01. However, because of scheduling constraints, and
because our priority was to obtain a large sample of HzRGs with 850 µm detections,
this limit was not always reached. Rather, the next target was observed as soon as an
apparent 5σ detection had been obtained at 850 µm.
The atmospheric optical depths τ850 , τ450 were calculated using the empirical CSO–
tau correlations given by Archibald et al. (2002), unless the values obtained through
skydips disagreed strongly, in which case those were used instead. The optical depth
τ850 varied between 0.14 and 0.38 with an average value of 0.26. The data were clipped
at the 4σ level to ensure accurate determination of the sky level, flat–fielded, corrected
for extinction, sky noise was removed after which they were co–added and clipped
at the 2.5σ level using the Scuba User Reduction Facility software package (SURF; Jenness & Lightfoot 1998), following standard procedures outlined in the SCUBA Photometry Cookbook1 . The concatenated data were checked for internal consistency using a
Kolmogorov–Smirnov (K–S) test and severely deviating measurements (if any) were
removed. Finally, flux calibration was performed using HLTAU, OH231.8 and CRL618
as photometric calibrators. The typical photometric uncertainty for our program is of
order 10–15 per cent, as estimated from our results on three sources (TN J0121+1320,
TN J1338−1942, and MG 2141+192) that were observed at two separate instances each.
This photometric accuracy is consistent with an estimated 10 per cent systematic uncertainty in the 850 µm flux density scale (see e.g., Papadopoulos et al. 2000; Jenness
1
The SCUBA Photometry Cookbook is available at http://www.starlink.rl.ac.uk/star/docs/sc10.htx/sc10.html
18
Chapter 2. Dust and star formation in distant radio galaxies
et al. 2001). Given that HzRGs appear to be located in submm overdense regions (e.g.,
Stevens et al. 2003) flux may have been lost due to chopping onto a nearby galaxy.
However this very unlikely to have affected more than a few sources.
Following Omont et al. (2001), Table 2 includes a column indicating the quality
of the observation. Good quality data is indicated by an ‘A’, whereas poor quality is
indicated by a ‘B’. Poor quality reflects bad atmospheric conditions (e.g. large seeing),
short integration time (< 2 sets of 50 integrations each), or poor internal consistency as
shown by the K–S test (i.e. the measurements were not consistent, but it was impossible
to determine which were the outliers. In such cases the average of all measurements
was used).
2.2 Potential contamination of the thermal submillimetre flux
Because all our objects are powerful radio galaxies, it is important to estimate any
synchrotron contribution to the observed submm band. We used flux densities from
the WENSS (325 MHz Rengelink et al. 1997), Texas (365 MHz; Douglas et al. 1996) and
NVSS (1.4 GHz; Condon et al. 1998) surveys to extrapolate to 350 GHz (850 µm) frequencies using a power law. For 53W069, we extrapolated from the 600 MHz and
1.4 GHz values in Waddington et al. (2000). We find that the synchrotron contribution
at 850 µm is negligible for most galaxies in our sample. Only for some objects from
A01 (and PKS 1354−17) would this require corrections larger than the 1σ uncertainties
in the 850 µm measurements.
Synchrotron spectra often steepen at high frequencies (e.g., A01; Athreya et al. 1997;
Andreani et al. 2002; Sohn et al. 2003), and linear extrapolation should be considered
an upper limit to the synchrotron contribution. A01 performed parabolic fits to account for the curvature of the radio spectrum. Using the midpoint between linear and
parabolic fits they find corrections larger than 1.5 mJy to the 850 µm flux densities in
only 6 cases. One could argue that parabolic fits are more appropriate, in which case
all corrections would be negligible.
Note that the radio measurements reflect the spatially integrated flux densities of
the sources. The radio cores have flatter spectra than the lobes and could dominate
at higher frequencies. However, they are usually faint and for USS sources even the
radio cores tend to have steep spectra (e.g., A01; Athreya et al. 1997) indicating that
contamination by the core is likely to be negligible as well.
Given these uncertainties, extrapolation from the radio regime to submm wavelengths is uncertain and is likely to result in an overestimate of the non-thermal contribution due to steepening of the radio spectrum. This is demonstrated for the case
of B3 J2330+3927, for which De Breuck et al. (2003) estimate a non-thermal contribution of ∼ 1.3 mJy at 113 GHz but measured a flux density < 0.3 mJy that seems to be
of thermal origin. Therefore we do not correct for a contribution to the submm continuum from the non-thermal radio emission, except for a few sources discussed below.
For these reasons (and following Willott et al. 2002) we have chosen also to use uncorrected fluxes from A01 in the remainder of this paper.
There are two exceptions that we exclude from our final sample. Following A01, we
reject B2 0902+34, as this source has a bright flat-spectrum radio core which could dominate the submm emission. We also reject the flat spectrum radio source PKS 1354−17.
Section 2. Sample Selection and Observations
19
It is significantly brighter than any of the other sources at 850 µm (S 850 = 20.5 ± 2.6 mJy),
but linear extrapolation from the radio regime shows that the non-thermal contribution
at 850 µm could be as large as 40 mJy and could easily account for all of the submm
signal.
Gravitational lensing may be important in some cases (Lacy 1999), resulting in enhanced submm fluxes. However, recent estimates (e.g., Chapman et al. 2002b; Dunlop
2002) show that this is limited to a small but significant fraction (3–5 per cent of sources
> 2) and that for most objects
with S850 > 10 mJy may have been boosted by a factor ∼
there is no evidence for strong gravitational lensing. Corrections for lensing must be
made on a case-to-case basis and are strongly model dependent. Since these corrections
are likely to be small, they have not been attempted for the present sample.
2.3 Dust template
As has been noted by many authors (e.g., A01; Hughes et al. 1997), choosing the dust
template is an important step in inferring the bolometric far infra-red luminosities
( LFIR ), star formation rates (SFR) and dust masses (Md ). A complication is that the
appropriate dust template may change over redshift due to changing dust properties
with the evolutionary states of the galaxies.
Throughout this paper we adopt single temperature, optically thin greybody emission for two sets of emissivity index β and temperature Td (see Dunne & Eales 2001;
Dupac et al. 2003,for possible concerns) as the functional parametrization for thermal
dust emission from high redshift sources. We choose β = 1.5 and Td = 40 K for comparison with other papers (e.g., A01; and see Dunne et al. 2000; Eales et al. 2003), and
also briefly investigate the effects of assuming β = 2.0 and Td = 40 K as seems reasonable for some hyperluminous IR galaxies (Td = 35 K; HyLIRGs Farrah et al. 2002) and
z > 4 quasars (Td = 40–50 K; Priddey & McMahon 2001; Willott et al. 2002). Measuring
the value of β for HzRGs directly would require observations at many more rest-frame
FIR wavelenghts than presented here. Increasing β or the dust temperature decreases
the inferred LFIR of high redshift sources relative to lower redshift sources for a given
flux density.
The fraction of absorbed UV/optical light, δSB , and possible departures from the
prototype Salpeter initial mass function, parametrized with δ IMF , are other uncertain
factors. Generally accepted approximations (see e.g., Papadopoulos et al. 2000; Omont
et al. 2001; De Breuck et al. 2003) for the dust mass, inferred FIR luminosity and star
formation rate are respectively:
Md =
Sobs DL2
,
(1 + z)κd (νrest )B(νrest , Td )
LFIR = 4π Md
8π h κd (ν )
c2 ν β
and
kT
h
Z
∞
0
β+4
κd (ν )B(ν, Td)dν =
Γ(β + 4)ζ (β + 4)Md ,
SFR = δIMF δSB ( LFIR /1010 L ) M yr−1 ,
20
Chapter 2. Dust and star formation in distant radio galaxies
with κd (ν ) ∝ ν β the frequency dependent mass absorption coefficient which modifies
the Planck function, B(ν, Td ), to describe the isothermal greybody emission from dust
grains, Γ the Gamma function, ζ the Riemann Zeta function, DL the luminosity distance and Sobs the observed flux density. The mass absorption coefficient is poorly
constrained (e.g., Chini et al. 1986; Downes et al. 1992; De Breuck et al. 2003; James
et al. 2002) and we conform to the intermediate value of κ d (375GHz) = 0.15 m2 kg−1
chosen by A01.
3 Observational Results
24 radio sources were observed. The results of the observations, the inferred rest-frame
850 µm luminosities L850 , total far-IR luminosities LFIR , and radio luminosities L3GHz
are summarized in Table 2. 12 of the HzRGs are detected at >3σ significance at 850 µm.
The median rms flux density of the observations is σ850 = 1.5 mJy with an interquartile
range of 0.8 mJy. Only B3 J2330+3927 may have been detected at 450 µm at a 2σ level
(S450 = 49.1 ± 17.7 mJy).
Particularly noteworthy is MG 2141+192. This source was detected by A01 at the
4.8σ -level at S850 = 4.61 ± 0.96 mJy, using the narrow filterset whereas we observed
S850 = 2.16 ± 1.10 mJy and S850 = 2.45 ± 1.58 mJy at two separate instances with the
wide filter set and did not detect the source. Similarly 4C 60.07 has been detected in
photometry mode at S850 = 11.5 ± 1.5, 17.1 ± 1.3 and in a jiggle map at 21.6 ± 1.3 (A01;
Papadopoulos et al. 2000; Stevens et al. 2003). These measurements are consistent to
within 2–3σ from the mean. However, naively, they could also be interpreted as signs
of submm variability. A01 and Willott et al. (2002) also found tentative evidence for
variability in the submm, but ascribed it to problems with sky subtraction for data obtained with the single-element bolometer UKT14 versus SCUBA. It is hard to see how
widespread star formation could result in submm variability on the time-scale of years.
Significant changes in LFIR might be easier to envisage as the result of UV variability
often seen in AGN and if the submm emission results from quasar heated dust. However, even in this scenario changes in the UV are expected to average over time in the
observed submm regime, unless the FIR emitting region is compact. While the typical
scale sizes for UV emission from the AGN are on parsec scales, the minimum extent
of the FIR emitting region must be about 1 kpc to match the observed luminosity and
dust temperature (e.g., Carilli et al. 2001). Submm variability, if real, is therefore hard
to explain if reprocessing by dust is the dominant mechanism for the FIR emission. If,
alternatively, the FIR emission would be non-thermal emission from the AGN, then the
emission should be unresolved in contrast to the observed extents of a few tens of kpc.
Moreover, observations indicate that AGN contribute at most 30 per cent of the FIR
luminosity at wavelengths longer than 50 µm, at least for HyLIRGs (Rowan-Robinson
2000; Farrah et al. 2002).
Possible explanations therefore may be that (i) for different chopping angles and
distances sometimes flux is accidentally lost due to companion galaxies in the off-beam
since the fields are overdense (Stevens et al. 2003), or (ii) that sometimes the AGN do
in fact contribute close to the maximum amount expected (note that the submm flux
for 4C 60.07 is centrally concentrated; Fig. 2 in Stevens et al. 2003), even though the
Section 4. Analysis
21
Figure 1 — Observed 850 µm flux density and 3σ upper limits versus redshift
of all 67 radio galaxies discussed in this
paper. The size of the arrows and errorbars correspond to 1σ rms.
starbursts still dominate. (iii) Finally of course there is still the possibility of pointing
errors, uncertainties in absolute flux calibration, and differences in atmospheric transparency that could shift the effective bandpass by a few GHz, which could make a
difference due to the very steep slope of the spectrum.
4 Analysis
In the following we discuss a sample of 67 radio galaxies (46/47 from A01, 23/24
from this paper, two sources were observed in both samples). This excludes the flatspectrum sources B2 0902+34 and PKS 1354−17 (see Section 2.2). For the statistical analysis we use the inverse variance weighted averages of the measurements of
MG 2141+192 (< S850 >= 3.3 ± 0.7 mJy) and 4C 60.07 (< S850 >= 14.4 ± 1.0 mJy) and for
MRC 1138−262 we prefer the value of S850 = 5.9 ± 1.1 mJy obtained by Stevens et al.
(2003) over our observation under adverse conditions. The median rms flux density
for this entire sample is σ850 = 1.1 mJy with a 0.24 mJy interquartile range.
Figure 1 shows that the observed submm flux densities and therefore the inferred
luminosities (see Table 2) at z > 3 vary significantly from object to object. For the
assumed dust template we find a range from LFIR < 4 × 1012 L for undetected targets
to LFIR ∼ 2 × 1013 L for detected sources. There are several viable scenarios to explain
this. First, if all the warm dust is heated solely by young stars, then L FIR is linked to
the SFR, implying that the SFR differs significantly between objects. Alternatively,
there may be a range in produced dust masses as substantial dust production may
take more than a billion years if low-mass stars are the principal contributors (e.g.,
Edmunds 2001). In this case the range in LFIR may reflect a range in starburst ages.
The recent detection of significant amounts of dust in the local supernova remnant
Cassiopeia A (Dunne et al. 2003), indicates that much faster evolving massive stars
Chapter 2. Dust and star formation in distant radio galaxies
22
Variable
Dependent
S850 µm
Dlin
L3 GHz
L850 µm
L850 µm
L850 µm
L850 µm
α
α
Independent
z
L3 GHz
z
L3 GHz
z
α
Dlin
L3 GHz
z
Percentage of
Data Censored
67% (52)%
0%
0%
67% (52)%
67% (52)%
67% (52)%
67% (52)%
0%
0%
Cox
0% ( 0%)
17%
1%
1% ( 1%)
0% ( 0%)
1% ( 1%)
56% (22%)
36%
0%
Significance (P)
Kendall
0% ( 0%)
34%
2%
7% ( 8%)
0% ( 0%)
1% ( 0%)
79% (89%)
20%
0%
Spearman
0% ( 0%)
32%
2%
10% ( 8%)
0% ( 0%)
1% ( 0%)
96% (78%)
19%
0%
Correlation
Present?
YES (YES)
NO
YES
MAYBE (MAYBE)
YES (YES)
YES (YES)
NO (NO)
NO
YES
Table 3 — Results of survival analyis for all 67 sources. The significance P is the probability of the
< 5 per cent, then the
variables not being correlated according to each test. If all three tests yield P ∼
< 5 per cent then a correlation is regarded as
variables are taken to be correlated. If only some yield P ∼
possible, but uncertain. Results are shown for both a 3σ and 2σ (between brackets) detection limit.
may be at least as important in producing dust. A considerable contribution from
high-redshift supernovae would significantly lower the required time-scales for dust
production and would favor a range in SFRs to explain the range in LFIR .
Despite the various uncertainties, the result that the far infrared luminosities are
high is robust. This has important implications for the starburst nature of these galaxies: inserting the values for LFIR that we find using either dust template confirms that
HzRGs are vigorously forming stars up to rates of a few 1000 M yr−1 and have dust
masses of a few times 108 M . This is consistent with the notion that they are in a
critical phase of their formation, forming the bulk of their stellar masses.
4.1 Statistical analysis
We have performed statistical tests to search for possible correlations of the submm
with other properties of the radio galaxies. Specifically, we investigated whether there
are correlations with redshift (as reported by A01), radio luminosity (as indicator of
AGN contribution), largest angular size of the radio source (as indicator of age), Kband magnitude (as indicator of stellar mass or star formation rate), Lyα flux (as a
possible indicator of starburst “fuel”), and UV polarisation (as indicator of the relative
contributions of starburst and AGN to the UV continuum). Below, we discuss these in
more detail.
Since the sample contains a large fraction (of order 50 per cent) of non-detections
(i.e. upper limits) we have conducted survival analysis tests similar to A01. Survival
analysis allows the mixing of detections and upper limits (“censored data points”) in
a statistically meaningful way thereby preserving as much information as possible.
The results of the survival tests are summarized in Tables 3 and 4. Table 3 represents
the results for the entire sample, whereas Table 4 only considers the 32 galaxies with
redshifts z > 2.5.
We have defined the subsample with redshift z > 2.5 to test for selection effects and
remove them from our sample. The well known and strong (see A01) radio powerredshift relation in flux limited samples is likely to be the origin of many apparent
correlations with redshift (see Table 3). Another example of selection effects is the
correlation found between L850 and radio spectral index α (Table 3). It reflects the
tight correlations between spectral index and redshift (part of our search criteria for
Section 4. Analysis
Variable
Dependent
S850 µm
Dlin
L3 GHz
L850 µm
L850 µm
L850 µm
L850 µm
L850 µm
L850 µm
α
α
Independent
z
L3 GHz
z
L3 GHz
z
α
Dlin
LLyα
Kmag
L3 GHz
z
23
Percentage of
Data Censored
53% (34%)
0%
0%
53% (34%)
53% (34%)
53% (34%)
53% (34%)
59% (41%)
59% (44%)
0%
0%
Cox a
95% (81%)
66%
7%
21% (33%)
88% (75%)
88% (99%)
13% ( 6%)
−
−
56%
2%
Significance (P)
Kendall
17% (25%)
8%
4%
33% (48%)
20% (34%)
81% (69%)
16% ( 7%)
86% (91%)
47% (33%)
21%
10%
Spearman
34% (38%)
9%
4%
46% (58%)
35% (52%)
99% (71%)
13% ( 6%)
4% ( 4%)
88% (72%)
11%
8%
Correlation
Present?
NO (NO)
NO
MAYBE
NO (NO)
NO (NO)
NO (NO)
NO (NO)
MAYBE (MAYBE)
NO (NO)
NO
MAYBE
Table 4 — Similar to Table 3. Results of survival analysis for 32 sources at redshifts z > 2.5.
a
Cox’s proportional hazard model only allows censoring in the dependent variable, therefore this test
could not be applied with the Lyα and K-band data.
Figure 2 — Histograms of 850 µm SCUBA detections with S/N > 2 (left) and S/N > 3 (right) versus the
total number of observed radio galaxies (grey) as a function of redshift. The detections from Archibald
et al. (2001) are shown as dashed, the detections from our program are shown in black. At z > 2.5 ∼
50–67 per cent of the galaxies are detected, as opposed to ∼ 15 per cent at z < 2.5. The bins have a width
of unit redshift and are centered at z = 1, 2, 3, 4, 5.
Chapter 2. Dust and star formation in distant radio galaxies
24
HzRGs) and between L850 and redshift. In the z > 2.5 sample this correlation almost
disappears even though there is a large range in both L850 and α, confirming that it
was spurious. An additional advantage of this subsample is the homogeneity of the
supporting data (Lyα fluxes and K-band magnitudes are understandably sparse for
lower redshift sources).
Below, we describe the details of the survival analysis and the criteria used to determine whether a correlation is present. In Section 5, we discuss the correlations individually.
Survival analysis
The application of survival analysis methods to astronomical data has been described in detail by Feigelson & Nelson (1985) and Isobe et al. (1986). We have made
use of the routines in the STSDAS package of IRAF2 (Tody 1993), which were modelled
after the ASURV package (Lavalley et al. 1992). Cox’s proportional hazard model,
the generalized Spearman’s rank order correlation coefficient, and the generalized
Kendall’s tau correlation coefficient test the null hypothesis that no correlation is present
in the sample. We adopt the convention that two variables are correlated if the chance
P of the null hypothesis being true is smaller than 5 per cent. While these tests should
give similar results they have different specific limitations and strong points. There< 5 per cent, and
fore, a correlation is considered to be reliable if all tests yield P ∼
< 5 per cent. Note that the Cox test
possible but unconfirmed if only some indicate P ∼
only allows censoring in the dependent variable and that the generalized Spearman’s
Rho routine is not reliable for small data sets (N < 30). SCUBA occasionally yields
negative flux densities. We defined the nσ upperlimit for those cases to be n × the rms
flux density. Similarly, the nσ upperlimit for a positive signal is defined as S + n× the
rms flux density, with S the observed signal.
5 Correlations between parameters
Tables 3 and 4 represent the results of the survival analysis as described above. We
will now discuss the motivation for each test and the implications of the correlations
(or lack thereof) found in order of importance.
5.1 Redshift dependent submillimetre properties
5.1.1 Flux density and relative detection fraction
Table 3 shows that the observed submm flux densities S850 are strongly correlated with
redshift. This is reflected in Figure 1 and 2. Figure 1 shows the observed 850 µm
flux densities against redshift. Figure 2 shows the number of radio galaxies that have
been observed in a particular redshift bin and the number of those which have been
detected given a 2σ or 3σ detection criterion. The success rate of our z > 3 program is
2
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the
Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
Section 5. Correlations between parameters
25
Figure 3 — Cumulative histograms
ΣN(z) for redshifts of submm sources
from the literature and the 67 radio
galaxies (solid line) discussed in this
paper.
The short-dashed and the
dotted line indicate submm detections
of radio galaxies versus non-detections
given a 3σ detection criterion, respectively. The detections have a higher
median redshift (z = 3.1) than the
parent sample (z = 2.5). The two
long-dashed lines represent ΣN(z) of
the 10 submm sources with spectroscopic redshifts found by Chapman
et al. (2003a) and using 9 additional
spectroscopic redshifts (Ivison et al.
1998; Ivison et al. 2000; Dey et al. 1999;
Soucail et al. 1999; Eales et al. 2000;
Ledlow et al. 2002; Chapman et al.
2002b,c; Aretxaga et al. 2003; Frayer
et al. 2003).
truly remarkable, and corroborates the conclusion of A01 that the detection fraction of
∼ 50–67 per cent at z > 2.5 is significantly different from the detection fraction of ∼ 15
per cent at z < 2.5. Removing the 4 brightest sources from the sample does not destroy
this relation.
5.1.2 Investigation of the difference between detections and non-detections
Given a 3σ detection criterion the average submm flux density of the 22 detected radio sources and 45 non-detections are < S850,≥3σ > = 6.60 ± 0.71 mJy and < S850,<3σ >=
0.81 ± 0.20 mJy respectively, while the average for the entire sample is < S 850,sample >=
2.71 ± 0.43 mJy. For a 2σ detection criterion the average submm flux density of the 32
detected galaxies and 35 non-detections are < S850,≥2σ >= 5.29 ± 0.60 mJy and < S850,<2σ >=
0.34 ± 0.19 mJy respectively.
Figure 3 shows cumulative redshift distributions Σ(z) for various subsets of the
radio galaxy sample together with spectroscopic redshifts for other submm samples
from the literature. As was suspected from Figure 2, the radio galaxies detected in the
submm follow a distribution that differs significantly from both the undetected sources
and the parent sample. The K–S test shows with P > 99 per cent confidence that using
a 2σ or 3σ detection criterion picks out the same population (both for the detections
and for the non-detections), whereas the chance that the detections and non-detections
are distributed similarly is P < 1 per cent.
Does the high median redshift (z = 3.1) of the detected sources (the parent sample
has z = 2.5) purely reflect the strong negative K-correction or does it reflect a change
in LFIR ? Figure 3 shows that the median redshift of the detections is higher than the
median redshift (z = 2.4) for SCUBA sources (Chapman et al. 2003a), while the redshift
distribution of the parent sample follows the SCUBA population closely. This argues
in favor of a different LFIR for detections and non-detections. What then causes the dif-
26
Chapter 2. Dust and star formation in distant radio galaxies
Figure 4 — The average rest-frame 850
µm luminosity versus redshift for 67
radio galaxies as estimated using the
Kaplan–Maier estimator for each redshift bin. The data points indicated
with open squares and circles were calculated assuming the same dust template as A01 (β = 1.5, Td = 40 K) for 3σ
and 2σ detection criteria respectively.
The solid symbols are similar but assume β = 2.0 instead. The data points
at z = 5 are 3σ upperlimits based on the
non-detection of TN J0924−2201.
ference in redshift distribution between detected HzRGs and the submm population?
There are at least two possible explanations. It may indicate that HzRGs are more massive, at the centers of protoclusters (Venemans et al. 2003), and therefore undergo a
faster evolution than ‘normal’ submm sources and finish the bulk of their formation
process early. Alternatively, it may reflect a selection effect as the requirement of a
faint radio counterpart prior to spectroscopic follow-up for the submm sample selects
against high redshift galaxies (Chapman et al. 2003a).
5.1.3 The increase of submm luminosity with redshift
Figure 4 shows the inferred 850 µm luminosity L850 as a function of redshift. For each
redshift bin L850 has been estimated using the Kaplan–Maier estimator, a survival analysis technique which tries to estimate the true distribution of the underlying population by incorporating both upperlimits and detections. We have computed the luminosities both using a dust template with β = 1.5 and β = 2.0. Adding our data to the
sample of A01 confirms that HzRGs have higher submm luminosities than lower redshift sources. However, there are significant differences with the findings from A01: (i)
L850 at low redshifts is higher than inferred by A01. This is because we have chosen,
not to correct for possible synchrotron contamination (see Section 2.3) (ii) There is weak
evidence for a turnover or leveling off at z > 4 (based on the lower flux densities of the
five galaxies with z > 4 compared to redshifts 3 < z < 4; see Fig. 1). This explains why
there is no strong correlation between L850 and z in the high redshift subsample (see
Table 4). (iii) A01 remarked that the increase in L850 becomes less pronounced if one assumes a dust template with β = 2. Because L850 at low redshifts is higher than in A01,
this effect is even stronger in our sample and we find that L850 may be rather constant.
The sources would still be highly submm luminous, implying huge dust masses and
star formation rates.
Section 5. Correlations between parameters
27
Figure 5 — Submillimetre luminosity
against radio power L3GHz for 32 radio
galaxies with z > 2.5. No strong correlation is apparent, indicating that heating of the dust by AGN is not a dominant process for radio galaxies. Symbols as in Figure 1.
5.2 The connection between submm and radio luminosity
Radio galaxies host luminous AGNs and for favorable geometries (e.g., dusty warped
disks) heating of dust by only a small fraction (≤ 20 per cent) of their UV radiation
could easily explain typical far-IR luminosities (e.g., Sanders et al. 1989). Therefore, an
important question is: do the AGN heat the large-scale dust significantly, i.e. are L radio
and LFIR correlated?
To answer this question we have estimated the rest-frame radio power at 3 GHz,
L3GHz . We have chosen this frequency because for the redshift range 1.1 < z < 7.2
this requires only interpolation between between 365 MHz and 1.4 Ghz. Of course this
assumes that the UV emission of the central source relates to its radio output (e.g.,
Willott et al. 1999; De Breuck et al. 2000).
While Table 3 shows that there is a possible correlation between L 850 and L3GHz over
the entire redshift range of our sample, this is probably an effect of the strong redshift
dependence of both L850 and L3GHz . The tests represented in Table 4 only take radio
galaxies with z > 2.5 into account and even though this subsample contains almost all
submm detections the correlation disappears. The scatterplot in Figure 5 is a graphical
representation of the same result. We don’t find evidence for dust heating by AGN,
in accordance with earlier findings (e.g., A01; Willott et al. 2002; Andreani et al. 2002)
but see Haas et al. (2003) for a contrasting view based on ISO observations of local
(most have 0.1 < z < 1.0) hyperluminous quasars. This analysis is likely to be an
oversimplification because the typical time-scales for starburst and radio activity differ
by an order of magnitude.
Chapter 2. Dust and star formation in distant radio galaxies
28
Source
3C356
3C368
3C324
4C40.36
4C48.48
4C23.56
MRC0316−257
TNJ0121+1320
TX1243+036
6C1908+722
4C41.17
TNJ2007−1316
TNJ1338−1942
z
1.079
1.132
1.206
2.265
2.343
2.483
3.130
3.516
3.570
3.532
3.798
3.830
4.100
P(%)
14
<1
12
7.3
8.4
15.3
<4
7
11.3
<5
< 2.4
<3
5
S850 (mJy)
< 4.8
4.1
< 4.4
< 3.9
5.1
< 4.7
< 8.9
7.5
< 5.6
10.8
12.1
5.8
6.9
Table 5 — UV/optical polarisation fractions
and submm fluxes for all radio galaxies for
which both have been observed. UV/optical
polarisation data are taken from Dey et al.
(1997), Vernet et al. (2001), and De Breuck et al.
in preparation, the submm data are from A01
and this paper.
Figure 6 — 850 µm flux S850 against
observed UV continuum polarisation
fraction. None of the highly polarized
sources are detected in the submm, indicating that low polarisation (indicative of a starburst) and high LFIR both
trace star formation.
Section 5. Correlations between parameters
29
5.3 An anti-correlation between submm flux and UV polarisation
Optical polarimetry studies can be used to search for direct signs of starburst activity
and determine the relative contributions of AGN and starburst to the UV continuum
(e.g., Dey et al. 1997; Vernet et al. 2001). If the starburst dominates the UV/optical
light, then the scattered (i.e. polarized) AGN light is diluted and one expects a low
polarisation fraction of the UV continuum and perhaps a correlation with the observed
submm flux densities.
Polarimetry results exist for 13 of the galaxies in our sample (Dey et al. 1997; Vernet
et al. 2001,De Breuck et al. in preparation). A summary of the polarisation fractions
and corresponding 850 µm flux densities is given in Table 5. We have plotted the observed submm flux density against UV continuum polarisation fraction in Figure 6.
No sources with high UV polarisation are detected in the submm. This is supported
formally by survival analysis: Kendall Tau gives a probability P = 3 per cent that there
is no correlation.
This result is consistent with the view that for HzRGs star formation and submm
emission are closely linked while any AGN contribution to the FIR is negligible. Tadhunter et al. (2002) also found some evidence for this. Two out of the three objects
which are starburst dominated in their sample of 2-Jy radio galaxies are also extremely
FIR luminous. Further support for low polarisation in starbursting systems comes
from the luminous submm source SMM J02399−0136 (Vernet & Cimatti 2001) which
shows only moderate (P ∼ 5 per cent) polarisation.
The weakness of the submm luminosity in highly polarized sources together with
the tentative correlation between Lyα and submm flux (see next section) is consistent
with the anti-correlation between Lyα luminosity and UV polarisation reported by
Vernet et al. (2001). They argue that Lyα photons are resonantly destructed by dust
and that scattering by the same dust results in a higher fractional polarisation. This
explanation, however, seems at odds with the lower submm flux densities in highly
polarized sources unless there would be significant amounts of dust that is too hot to
be detected in the submm.
5.4 Submillimetre and Lyα flux
Since many HzRGs are embedded in giant Lyα haloes (e.g., McCarthy 1993; van Ojik
et al. 1996; Reuland et al. 2003) it would be interesting to see if one can relate the
amount of available star formation ‘fuel’, as estimated roughly by the spatial size, or
luminosity, of these emission line haloes and the amount of HI as probed by Lyα absorption to the observed rest-frame FIR luminosities. Alternatively, one might expect
a strong anti-correlation due to destruction of Lyα emission by dust.
Many HzRGs are identified based on their Lyα line and good data on the observed
Lyα fluxes is available in most cases. There is a strong selection effect, however, since
sources with low Lyα flux densities are less likely to be recognized as HzRGs. Interestingly, Table 4 and Figure 7 indicate that a correlation may be present. While this
relation is tentative only, it offers perspectives for future programs. Better data should
make it possible to investigate whether there is a possible relationship between the
dust content and Lyα to C IV or N V emission line ratio, as indicators of metalicity
Chapter 2. Dust and star formation in distant radio galaxies
30
Figure 7 — S850 µm against observed Lyα line flux (left) and inferred luminosities L 850 against LLyα
(right). Tentative evidence for a correlation is apparent. The galaxies brightest in Lyα seem to have
higher submm flux densities, possibly indicating that galaxies with richer gaseous environments (as
traced by Lyα) also host more massive starbursts. Symbols as in Figure 1.
(c.f. Vernet et al. 2001). This would be especially interesting in the light of findings
by Hamann & Ferland (1999) and Fan et al. (2001) that QSOs show (super)solar abundances out to the highest redshifts and must already have undergone significant star
formation. A comparison with the more readily studied HzRG host galaxies would be
crucial to better understand this result.
The detection of both Lyα and dust in many HzRGs is intriguing because already a
small amount of dust can extinguish Lyα radiation efficiently. The detection of Lyα in
the submm sources of Chapman et al. (2003a) is equally surprising. One explanation
may be that Lyα emission and dust are located predominantly in different locations
(e.g., a merger between one dusty and one less dusty component). Some evidence for
this comes from 4C 60.07 at z = 3.8 in which the spatially resolved dust continuum
emission (Papadopoulos et al. 2000) is anti-correlated with the Lyα emission (Fig. 5 in
Reuland et al. 2003). Smail et al. (2003) also report evidence for an offset between far-IR
and UV emitting regions in the z = 2.38 starburst galaxy N2 850.4.
5.5
L850 and linear size
Willott et al. (2002) found an anti-correlation between linear size and 850 µm luminosity for their sample of z ∼ 1.5 radio loud quasars. This effect was attributed to a
possible relation between the jet-triggering event and a short-lived starburst or quasarheated dust in QSOs. No such correlation was found for a matched subset of the A01
sample centered at the same redshift. We have briefly investigated whether such a correlation exists in the present sample. Figure 8 shows the observed submm flux plotted
against the projected linear size. No correction for possible differences in inclination
angle was made because such corrections are expected to be relatively small among
radio galaxies (even when comparing radio galaxies with quasars projection effects re-
Section 5. Correlations between parameters
31
Figure 8 — 850 µm luminosity L850
against the radio source projected linear size Dlin . There is no evidence for
a strong anti-correlation as was found
for radio loud quasars by Willott et al.
(2002). Symbols as in Figure 1.
sult in only a factor ≈ 1.6 difference in projected linear size for θ trans = 53 ◦ ; Willott et al.
2000, 2002). Our analysis does not support such a strong relation and if one interprets
linear size as an estimate of the age of the radio source (reasonable if one assumes
that the environments and jet powers do not change significantly; see e.g., Blundell,
Rawlings & Willott 1999) , then this would be consistent with the scenario sketched by
Willott et al. that the correlation found for QSOs is indicative of short starbursts while
the starbursts in HzRGs have longer time-scales and could be forming the bulk of the
stellar populations. Despite the absence of a clear correlation, some relation between
age of radio source and starburst might have been expected if the black-hole and stellar
bulge grow in a symbiotic fashion (e.g., Williams et al. 1999). However, in realistic scenarios many complicating factors can be envisaged such as the significantly different
time-scales involved. Growth of the radio source may first induce star formation by
compressing molecular clouds (Begelman & Cioffi 1989; Bicknell et al. 2000) and may
then actually signal the end of the starburst by clearing out all the cool gas from the
central region (Rawlings 2003).
5.6 Submillimetre and near-IR emission
As discussed by Isaak et al. (2002), for QSOs one might naively expect a correlation
between the submm and optical flux densities. This correlation might arise regardless
of whether stars or a buried AGN are responsible for heating of the dust since black
hole mass scales with stellar bulge mass. Priddey et al. (2003) note that the picture is
likely to be more complex and that any correlation might be smeared out due to differences in relative timing between AGN fueling and starburst, or varying dust-torus
geometries. Radio galaxies might be better suited for such studies, since according
to orientation based unification schemes (e.g., Barthel 1989) the AGN are obscured by
a natural coronograph and rest-frame B-band luminosity therefore is a fairly reliable
32
Chapter 2. Dust and star formation in distant radio galaxies
Figure 9 — S850 µm against observed Kband magnitude. No correlation is apparent, and no relation between LFIR
and stellar mass/optical star formation
rate of the host galaxy can be established. Symbols as in Figure 1.
measure of the mass of the stellar population or starburst activity. For this reason we
have correlated the K-band magnitudes (samples rest-frame B,V for z > 3) with the
submm flux densities. As can be seen from Table 4 and Figure 9 no such correlation
is apparent. This remains true, if we apply a K-correction to the near-IR magnitudes.
However, the dataset is rather inhomogeneous and more sensitive data in both the
submm and near-IR are required to truly rule out any correlation. Sensitive multiband observations could be used to (i) construct a redshift independent estimate of
the line-free rest-frame optical continuum for comparison with LFIR ([[O III]] and Hα
can sometimes dominate the K-band e.g., 4C 30.36, 4C 39.37;Egami et al. 2003), and
(ii) check whether FIR emission is stronger for sources with higher intrinsic reddening
(following e.g., Calzetti 1997; Adelberger & Steidel 2000; Seibert et al. 2002).
5.7 Summary
From our analysis we consider the following results the most interesting:
1. S850 and z are strongly correlated. Whether this is true also for L FIR and z depends
on the assumed dust template.
2. L850 and radio power L3GHz do not correlate strongly, indicating that AGN do
not dominate the submm emission (either through synchrotron contamination or
dust heating by the UV continuum).
3. S850 and Lyα appear weakly correlated. While this is not very convincing, the
interesting result is the absence of a strong anti-correlation as expected naively
from the destruction of Lyα emission by dust.
4. S850 and UV polarisation fraction appear anti-correlated. This is expected if the
UV continuum of the buried AGN does not contribute significantly to the dust
heating.
Section 6. A comparison of radio galaxies with QSOs
33
Figure 10 — The 850 µm detection (left S/ N > 2; right S/ N > 3) fractions of radio galaxies as function
of redshift (solid line). The fractions rise up to a redshift of z = 4. For comparison we also plot the
detection fraction for observations of radio quiet QSOs at z = 1 – 5, and radio loud quasars at z ∼ 1.5
(after matching the sensitivity to 3 mJy rms) at 850 µm (dashed line; Priddey et al. 2003; Isaak et al. 2002;
Willott et al. 2002) and 1.25 mm (dotted line; Omont et al. 2001, 2003; Carilli et al. 2001; Petric et al. 2003;
Bertoldi et al. 2003). The detection rates for the QSOs appear to follow the same trend as for the radio
galaxies, but the QSO datapoint at z = 5 seems indicative of a turnover (the z = 5 point for the HzRGs is
based on only 1 galaxy, but taking all z > 4 HzRGs yields a similar result). The datapoint at z = 6 could
indicate a detection fraction larger even than what is observed for z = 4. This might reflect, the extreme
properties of those sources.
6 A comparison of radio galaxies with QSOs
The optically thin (sub-)mm emission of starbursts should be largely independent of
viewing angle. Orientation based unification schemes for AGN (e.g., Barthel 1989)
therefore suggest that we can combine observations of radio galaxies with those of
QSOs, allowing us to study the evolution of star formation in their host galaxies over
a larger redshift range. Extensive surveys of high-redshift (mostly radio-quiet) QSOs
have been published recently (Carilli et al. 2001; Omont et al. 2001, 2003; Isaak et al.
2002; Priddey et al. 2003; Petric et al. 2003; Bertoldi et al. 2003). We have combined
their results and compare those with our sample.
Figure 10 shows the detection fraction as a function of redshift for the QSO and
radio galaxy samples. In many redshift bins only a small number of detections and
non-detections is responsible for estimating the success rates for the present effective
flux limits. We have estimated the range over which the ‘true success rate’ could vary
such that there would be a 68 per cent chance of measuring the observed detection fraction. This range is indicated by the errorbars. Figure 11 shows the average (sub-)mm
flux density for the various samples as a function of redshift.
Reassuringly, the trends for the QSOs and radio galaxies shown in these figures
mimic each other. The average observed (sub-)mm flux density rises to redshifts of
z ∼ 4. For higher redshifts there is some evidence for a decline (see the discussion in
Section 5.1.3 and the z = 5 data point of the MAMBO QSO obervations). However, the
34
Chapter 2. Dust and star formation in distant radio galaxies
Figure 11 — The average measured
(sub-)mm flux density binned in redshift has been plotted for QSOs and radio galaxies. The squares represent observations of radio galaxies at 850 µm,
the triangles QSOs at 850 µm, while the
circles represent observations of QSOs
at 1250 µm multiplied by a factor of
2.5 for easier comparison. The datapoint at z ≈ 1.5 represents the radio
loud quasars from Willott et al. (2002).
The trends with redshift are similar to
what is seen in Figure 10 and confirm
the steady rise from z = 1 to z = 4 for
the galaxies.
observations of z > 4 QSOs would also be consistent with a fairly constant or even a
rise in the detection fraction and average FIR luminosity out to the highest redshifts.
This depends rather critically on the z ∼ 6 QSOs, which are likely to be atypical even
for QSOs.
The average flux density and detection rate of the QSOs is comparable to that of
the HzRGs, even though the HzRG surveys are more sensitive than the QSO surveys
by a factor of approximately 3. This, coupled with the assumption that quasars are
on average more luminous than radio galaxies in AGN unification schemes based on
receding torus models (e.g., Simpson 2003), may be viewed as further support for an
AGN related component in QSOs, similar to what was found for radio quasars (Andreani et al. 2002; Willott et al. 2002). Note that the quasars discussed by Willott et al.
(2002) are exceptionally bright compared to the mostly radio quiet QSOs represented
in Figs. 10,11.
Due to a lack of carefully matched surveys fundamental differences between type I
and type II AGN and radio-loud versus radio-quiet targets cannot be properly investigated yet, although slowly progress is being made.
7 Summary
We have presented SCUBA observations of 24 radio galaxies and compared those with
earlier results of 47 radio galaxies from the survey by A01. We confirm that HzRGs
are massive forming galaxies, forming stars up to rates of a few thousand M yr−1 and
that there is no strong evidence for a correlation with radio power. Further evidence
for a predominantly starburst nature of the far-IR emission comes from the striking
anti-correlation between submm flux density and UV polarisation (Fig. 6).
In agreement with A01 we find that submm detection rate appears to be primarily
Section 7. Summary
35
a function of redshift. If this is interpreted as being due to a change in the intrinsic
far-IR luminosity, it would be consistent with a scenario in which the bulk of the stellar
population of radio galaxies forms rapidly around redshifts of z = 3 − 5 after which
they are more passively evolving (c.f. Best, Longair, & Röttgering 1998). We also
find that the median redshift of the HzRGs with SCUBA detections (z = 3.1) is higher
than the median redshift of the submm population (z = 2.4; Chapman et al. 2003a). In
the current picture of hierarchical galaxy formation, this could be interpreted as that
HzRGs are more massive galaxies, which are then thought to begin their collapse at
earlier cosmic times and evolve faster and finish the bulk of their formation process
earlier. Alternatively, it could indicate that higher redshift submm sources are being
missed due to the requirement of a radio counterpart prior to spectroscopic follow-up.
HzRGs have accurately determined redshifts and host identifications and are thought
to be the most massive galaxies at any epoch (De Breuck et al. 2002). Therefore, HzRGs
are a key population for studies of galaxy formation in the early universe, allowing detailed follow-up mm-interferometry observations to study their dust and gas content.
Currently, they offer the best way to obtain reliable dynamic masses for a significant
number of massive high redshift galaxies. These HzRGs would be especially suited to
constrain any evolution in galaxy mass with redshift, study changes in evolutionary
status, gas mass, and the starburst-AGN connection. If all of these turn out to have
masses larger than 1011 M , then this could have important consequences (depending
on rather uncertain correction factors for the fraction of similar galaxies for which the
black hole is dormant) for our understanding of galaxy formation, because only few
such massive galaxies are expected at such high redshifts (Genzel et al. 2003).
Acknowledgments
We thank Chris Willott and Steve Rawlings for useful discussions and the anonymous
referee for helpful suggestions to improve the paper. We gratefully acknowledge the
excellent help of the JCMT staff and ‘flexible’ observers who collected data for our
program. In particular the help of Remo Tilanus was indispensable. The authors wish
to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we
are priviledged to be guests. The JCMT is operated by JAC, Hilo, on behalf of the
parent organizations of the Particle Physics and Astronomy Research Council in the
UK, the National Research Council in Canada and the Scientific Research Organization
of the Netherlands. The work of M.R. and W.v.B. was performed under the auspices
of the U.S. Department of Energy, National Nuclear Security Administration by the
University of California, Lawrence Livermore National Laboratory under contract No.
W-7405-Eng-48.
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Chapter 3
An obscured radio galaxy at high redshift
Michiel Reuland, Wil van Breugel, Huub Röttgering, Wim de Vries, Carlos De Breuck and
Daniel Stern, The Astrophysical Journal Letters, Vol. 528, p. 71, 2003
Perhaps as many as 10% of high redshift radio galaxy (HzRG; z > 2) candidates
that are selected using an Ultra Steep radio Spectrum (USS) criterion fail to show
optical emission (continuum, lines) in deep Keck exposures. Their parent objects
are only detected in the near-IR and are probably heavily obscured and/or at very
high redshift. To search for signatures of dust and help constrain the nature and
redshifts of these “no-z” radio galaxies, we have conducted a program of submillimeter and millimeter observations. Here we report the first results of a detailed
study of one of these objects, WN J0305+3525.
WN J0305+3525 appears associated with a small group of K ∼ 21 − 22 objects and
is strongly detected at both 850 µm and 1.25 mm. On the basis of its faint Kband magnitude, spectral energy distribution (SED) and other evidence we estimate that the radio galaxy is probably at a redshift z ' 3 ± 1. This would make
WN J0305+3525 a radio-loud Hyper Luminous Infrared Galaxy (L FIR ∼ 1013 L )
similar to, but more obscured than, other dusty radio galaxies in this redshift
< 1 00. 9)
range. This, together with the absence of Lyα emission and compact (θ ∼
radio structure, suggests that WN J0305+3525 is embedded in a very dense, dusty
medium and is probably at an early stage of its formation.
1 Introduction
H
redshift active galactic nuclei (AGNs) have gained renewed interest as cosmological probes with the findings that (i) there is a good correlation between
the masses of the stellar bulges of galaxies and their central black holes suggesting a
causal connection in their formation (e.g., Magorrian et al. 1998) and (ii) that a population of high redshift heavily obscured AGNs seems the best candidate to account for
a substantial fraction (30−40%) of the 5−10 keV cosmic X-ray background (XRB) (e.g.,
Hasinger 2002; Stern et al. 2002; Willott et al. 2002).
The most vigorously star forming galaxies radiate strongly in the far-IR (Sanders &
Mirabel 1996). Sub-mm observations show that as many as 50% of z > 3 HzRGs are
“Hyper Luminous Infrared Galaxies” (HyLIRGs, LFIR ∼ 1013 L ; Chapter 2; Archibald
et al. 2001; Reuland et al. 2004). While some HyLIRGs are powered by an obscured
AGN, there also exist many in which the starburst dominates (Rowan-Robinson 2000).
The detection of strong rest-frame UV absorption features show that there are hot
young stars at z ∼ 3.8 in at least one case (Dey et al. 1997), and CO mm-interferometry
IGH
39
Chapter 3. An obscured radio galaxy at high redshift
40
Figure 1 — Keck/NIRC K-band image with 4.85 GHz VLA radio contours
(De Breuck et al. 2000) overlaid of the
HzRG candidate WN J0305+3525. The
greyscale image has been smoothed to
a resolution of 0 “. 7 and the contour levels are 0.25, 0.5, 1, and 2 mJy beam−1 .
Note the multiple components. The
bright object to the SW is a spectroscopically confirmed star. The circle with 3 00
radius represents the nominal 3σ astrometric uncertainty for the centroid of
the 850 µm emission and dashed lines
indicate the LRIS slit positions.
2
3
4
PA 65
PA 0
1
PA 50
*
studies show that star formation occurs galaxy wide over scales of up to ∼ 30 kpc
(Papadopoulos et al. 2000). These observations provide direct evidence that powerful HzRGs are massive galaxies forming stars at rates as high as 2000 M yr−1 . From
the near-IR Hubble K − z diagram HzRGs are known to be the most luminous galaxies at any redshift up to z ∼ 5.2 (De Breuck et al. 2002,hereafter DB02). Furthermore,
actively-accreting supermassive black holes (MBH ∼ 109 M ; Lacy et al. 2001) are required to power radio sources. Therefore, HzRGs are excellent targets to study the
formation and coevolution of the most massive galaxies and their black holes.
1400MHz <
De Breuck et al. (2001) found that 36% of a sample of 33 USS (α 325MHz
∼ −1.30;
α
Sν ∝ ν ) selected radio sources with spectroscopic information was at redshifts z > 3.
A surprisingly large fraction (24%) of these USS sources did not show emission lines
even in moderately long exposures (1−2 hrs) at Keck. Of these latter sources, 3 (or
9% of the entire sample) did not show optical continuum emission down to a limit of
R ∼ 25 and were detected only at near-IR (K-band) wavelengths.
The nature of these “no-z” objects is unclear. As discussed by De Breuck et al. one
can think of the following possible explanations: objects which do show optical continuum (i) may be in the “redshift desert” (1.5 < z < 2.3) where strong emission lines
fall outside the observable window or (ii) may be pulsars; objects without optical con> 7) that
tinuum (iii) may be highly obscured AGNs or (iv) be at such high redshift (z ∼
Lyα is shifted out of the optical passband. In the latter two cases they may be young
HzRGs in an exceptionally vigorous stage of their formation, and one would expect
large amounts of dust associated with them. To search for this we have conducted a
program of sub-mm and mm observations of these “no-z” radio galaxies. Here we
report the first results of a detailed study of one such object, WN J0305+3525.
To facilitate comparison with other papers we adopt an universe with ΩM = 1.0,
ΩΛ = 0, and H0 = 50 km s−1 Mpc−1 unless stated otherwise. The angular scale at z = 3
Section 2. Observations and Results
Figure 2 — Same as Figure 1, but
for the greyscale representation of
the Keck/NIRC J-band image of
WN J0305+3525. The image has been
smoothed to a resolution of 0 “. 7.
Objects 2 and 4 which were visible in
K-band remain undetected. This image
is similar to the I-band image (not
shown).
41
2
3
4
1
*
is then 7.3 kpc arcsec−1 and the look-back time is 11.4 Gyr, or 88% of the age of the
universe.
2 Observations and Results
2.1 Selection and Keck Imaging
VLA observations by De Breuck et al. (2000) show that WN J0305+3525 is a compact
1.4GHz
< 1 00. 9) steep spectrum (α325MHz
(θ ∼
= −1.33) radio source with flux density S1.4GHz =
15.8 mJy. As part of our program aimed at finding z > 3 radio galaxies among USS
sources DB02 obtained a 15 min I-band image with 0 “. 7 seeing and a 38 min K-band
image with 0 “. 55 seeing at Keck. We obtained a 58 min Keck J-band image on UT
00
2000 January 30 (seeing ∼ 0 . 6) with the Near Infrared Camera (NIRC; Matthews &
Soifer 1994), which was reduced following DB02. Formal 5σ detection limits in 2 00
diameter circular apertures are Ilim = 23.37, Jlim = 23.15, and Klim = 22.61. Astrometry for the I-band image was determined using the USNO-A2.0 catalog (Monet 1998)
giving a formal uncertainty of 0 “. 15. The absolute rms uncertainties with respect to
the international celestial reference frame of the catalog and radio image are 0 “. 2 and
0 “. 25 respectively (e.g., DB02). The small near-IR images were registered to the 6 0 × 7 0
I-band image to better than 0 “. 05 rms, resulting in relative astrometry between the
00
near-IR and radio images accurate to 1σ ∼ 0 . 4 rms.
Figure 1 shows the K-band image with 5 GHz radio map overlaid. It reveals fuzzy
structure with multiple components within 2 00 −3 00 of the radio source. Note that none
of the objects appears directly associated with the radio emission itself. Table 1 summarizes the observed properties of these objects. Objects 2 and 4 remain undetected in
the I and J-band images (Fig. 2).
Chapter 3. An obscured radio galaxy at high redshift
42
Source
1
2
3
4
Ensemblec
850 µm
4.85 GHzd
SW Stellar Object (∗)
R.A. (J2000)
3:05:47.30
3:05:47.40
3:05:47.56
3:05:47.56
DEC (J2000)
+35:25:11.0
+35:25:15.6
+35:25:14.4
+35:25:13.0
3:05:47.38
3:05:47.42
3:05:47.05
+35:25:15.0
+35:25:13.4
+35:25:11.3
Ia
22.88 ± 0.15
> 23.37
23.16 ± 0.19
> 23.37
23.41 ± 0.92
Ja
22.03 ± 0.20
> 23.15
22.06 ± 0.20
> 23.15
21.07 ± 0.18
Ka
21.16 ± 0.19
22.18 ± 0.33
21.30 ± 0.20
21.44 ± 0.24
20.71 ± 0.24
(I − J)b
0.9
−
1.1
−
>
∼ 2.3
(J − K)b
0.9
> 1.0
∼
0.8
>
∼ 1.7
0.4
19.61 ± 0.01
18.50 ± 0.04
18.36 ± 0.05
1.1
0.1
Table 1 — Optical, near-IR, 850 µm and radio parameters of WN J0305+3525.
a
Magnitudes have been corrected for Galactic reddening using the Cardelli, Clayton, & Mathis (1989)
extinction curve with A V = 0.85 as determined from the IRAS 100 µm maps of Schlegel, Finkbeiner, &
Davis (1998) and were measured in 2 00 diameter circular apertures, unless noted otherwise.
b
Calculated using formal 5σ detection limits in the aperture (Ilim = 23.37, Jlim = 23.15, Klim = 22.61) if
fainter than those. c Magnitudes have been measured in a 6 00 diameter circular aperture centered on
object 3 also encompassing objects 2 and 4.
d
Position from De Breuck et al. (2000)
2.2 JCMT and IRAM Observations
We observed WN J0305+3525 on UT 1999 December 6 and 7 at 850 µm and 450 µm
with the Submillimetre Common-User Bolometer Array (SCUBA; Holland et al. 1998)
at the James Clerk Maxwell Telescope in stable atmospheric conditions τ 850 ∼ 0.29 −
0.32. We performed two sets of 50 integrations each night using the 9-point jiggle
photometry mode while chopping 45 00 in azimuth at 7.8 Hz. Pointing was better than
2 00 . Flux calibration was performed on Saturn, HLTAU, and CRL618 and we adopted a
gain C850 ∼ 182 mJy beam−1 V−1 . The data were reduced following procedures outlined
in the SCUBA Photometry Cookbook1 . Combining the data from both nights yields an
average 850 µm flux density of 12.5 ± 1.5 mJy, while at 450 µm we measured 21 ±
19 mJy and did not detect the object. The FWHM of the beam at 850 µm for SCUBA
00
is θFWHM = 14 . 5. We imaged WN J0305+3525 with SCUBA on UT 2000 February 26,
27, and 28, reaching ∼ 1.5 mJy rms, to obtain a more accurate position for the sub00
mm source. The position of the 850 µm centroid α = 03h 05m 47 m. 38, δ = +35 ◦ 25 0 15 . 0
(J2000) with estimated 3σ astrometric uncertainty of ± 3 × θ FWHM /(2 × S/ N) ∼ 3 00 (following Serjeant et al. 2002) is indicated in Figure 1.
WN J0305+3525 was also observed at 1.25 mm using the 37 channel MPIfR Bolometer Array (MAMBO II; Kreysa et al. 1998) at the IRAM 30 m telescope on UT 2000 July
18. We performed 11 symmetric “ON-OFFs” with a chop-nod distance of 45 00 , each
with 20 subscans of 10 s per subscan. The atmospheric extinction τ 1.25 varied between
0.16 and 0.34 and pointing corrections were less than 2 00 . The calibration factor of 13.3
counts mJy−1 was estimated from on-offs on Uranus and Saturn. Using MOPSI with
sky-noise filtering we detect the object at 4.17 ± 0.56 mJy.
2.3 Keck Spectroscopy
We have attempted to measure the redshift of WN J0305+3525 using the Low-Resolution
Imaging Spectrometer (LRIS; Oke et al. 1995) with the 150 ` mm−1 grating blazed at
1
The SCUBA Photometry Cookbook is available at http://www.starlink.rl.ac.uk/star/docs/sc10.htx/sc10.html
Section 3. Discussion
43
7500 Å and 1 “. 5 wide slit resulting in a spectral resolution of 15 Å and wavelength coverage of 4100 − 10000 Å. Between exposures we shifted the object by 10 00 along the slit
to facilitate fringe removal in the red parts of the CCD. The slit positions are indicated
in Figure 1. A 3 × 30 min observation at PA=50 ◦ on UT 1999 December 20 through
targets 1 and 3 did not show emission lines. On UT 2000 February 1 we first observed
2 × 30 min at PA=65 ◦ through target 3, the formal radio source position, and the SW
stellar object (∗). No emission lines were found, but the latter object was identified
as a Galactic M giant. A second set of similar exposures at PA=0 ◦ through targets 3
and 4 did not show emission lines either, but we may have detected faint continuum
(Fλcont ≈ 2 ± 3 × 10−19 erg s−1 cm−2 Å−1 ) redward of λ ≈ 6300 Å from object 3. Object 2
was not observed spectroscopically, but it is unlikely that it would have been detected
given its faintness. Assuming that Lyα would fall between 5100 Å and 8900 Å where
the sky is well behaved and assuming a line width ∼30 Å (e.g., Dey et al. 1997), we
derive a 3σ upper limit to the Lyα flux density of 3 × 10−17 erg s−1 cm−2 .
3 Discussion
3.1 Identification and Redshift Estimate
What are the chances that the sub-mm source, the radio source and the K-band ob> 10 mJy number
jects are all unrelated? Sub-mm surveys have shown that the S 850 µm ∼
density is less than 100 per square degree (Hughes et al. 2002). Radio surveys show
> 15 mJy per square degree (de
that there are less than 12 radio sources with S1.4GHz ∼
00
Vries et al. 2002). Within the sub-mm error circle (3 radius; by far the largest of the
> 10 mJy sub-mm source and
three), the chance of finding a chance superposition of a ∼
−4
>
a ∼ 15 mJy radio source is negligible (< 2.3 × 10 ). The chance of finding three unrelated K < 22 mag objects within the error circle is less than 10% given a number density
∼ 2 × 105 per square degree (Djorgovski et al. 1995). Despite the offsets between the
K-band objects and the radio source there are good reasons to believe that they are related. Many USS selected radio galaxies consist of multiple components (van Breugel
et al. 1998). Furthermore, in most HzRGs the radio AGN are obscured (Reuland et al.
2003) and there are other cases where the AGN appears to be located off center from
the bulk of the parent galaxy (e.g., 3C294; Quirrenbach et al. 2001).
Without a spectroscopic redshift we can use only indirect arguments to estimate the
redshift of WN J0305+3525 such as the K − z diagram, the shape of the SED and the
near-IR colors. We cannot use the radio to sub-mm index as a redshift indicator (Carilli
& Yun 1999) as WN J0305+3525 is radio loud. The K − z diagram as determined for
HzRGs (DB02) seems to hold also for the sub-mm population (Dunlop 2002). If we
take a total magnitude of K ∼ 21 to be representative for WN J0305+3525 (Table 1),
<z∼
< 7.
then we estimate from the K − z diagram that WN J0305+3525 is at 3 ∼
< 6 with redshifted
Comparing the (sub)-mm ratios S1250 / S850 = 0.33 and S450 / S850 ∼
models of dusty starforming galaxies (c.f. Fig. 3 in Hughes et al. 1998) results in red<z∼
< 6 and z > 1 − 2 respectively. Moreover, almost all USS radio
shift estimates of 3 ∼
galaxies that are detected in the sub-mm are at redshifts z > 2.5. Figure 3 represents
the observed SED of WN J0305+3525 and shows that the (sub-)mm emission is of thermal origin and effectively rules out that the object is a pulsar. A starburst SED with
44
Chapter 3. An obscured radio galaxy at high redshift
Figure 3 — Radio to near-IR SED of
WN J0305+3525 using data from
the VLA (De Breuck et al. 2000),
IRAM, JCMT, and Keck (squares
and 5σ upper limit for 450 micron point). Extrapolation of the
steep radio spectrum shows that the
contribution of the non-thermal radio emission is negligible at submm wavelengths. For comparison
we overplot a schematic SED representing local ultraluminous galaxies, shifted to z = 3 (Lutz et al.
2001,H0 = 75 km s−1 Mpc−1 ).
<z∼
< 4 would fit the data closely at both the near-IR and (sub-)mm points.
1∼
The colors of the K-band objects are too uncertain to usefully constrain the redshifts
using stellar evolution models (Stevens & Lacy 2001). The possible detection of faint
continuum redward of λ ≈ 6300 Å in object 3 is consistent with the object being at high
redshift: it could be featureless continuum redward of a Lyα forest region at z ∼ 4.2.
Based on the above arguments we conclude that there is very good circumstantial
evidence that WN J0305+3525 is at high redshift, probably at z ' 3 ± 1.
3.2 Implications
If WN J0305+3525 is indeed at z ' 3 then its restframe FIR properties are L FIR ∼ 1013 L ,
implying Mdust ∼ 1.5 × 108 M and star formation rate ∼ 1500 M yr−1 for standard
dust models and cosmology (c.f. 4C 60.07; H0 = 75 km s−1 Mpc−1 ; Papadopoulos et al.
2000). This is comparable to the radio-“quiet” sub-mm source Lockman 850.1 which
<z∼
< 4, based on its sub-mm/mm SED (Lutz et al. 2001),
has been estimated to be at 2 ∼
as well as other, radio-“loud” galaxies in this redshift range. Compared with tradi< 1042.3 erg s−1 ) is relatively weak given its radio
tional HzRGs, the Lyα emission (LLyα ∼
power (P325MHz ' 1035.1 erg s−1 Hz−1 ), but still within the large observed scatter (c.f. Fig.
10 in De Breuck et al. 2000). Thus WN J0305+3525 appears to be a heavily obscured
radio-loud luminous sub-mm source. We consider two options that could explain the
strong obscuration.
Recently, Greenberg & Shen (2000) and Todini & Ferrara (2001) suggested that dust
particles in high redshift galaxies may be predominantly smaller than in local starbursting systems. The argument is based on a different origin of the dust: at high
redshifts there has been little time for M giants (the primary sources of local dust) to
appear, and most early dust would be produced by M supergiants and supernovae
instead. The increase of the sub-mm detection rate with redshift for radio galaxies
Section 3. Discussion
45
(Chapter 2; Archibald et al. 2001; Reuland et al. 2004) might then be interpreted as being due to changing dust properties with redshift. Smaller dust particles lower the
estimated dust masses and star formation rates and, because they are more efficient
UV absorbers, this might explain why some of the most massive starforming galaxies
are not seen in the optical at all.
Another reason that WN J0305+3525 is heavily dust enshrouded might be related to
its young evolutionary stage. This is supported by the compactness of the associated
radio source for WN J0305+3525 and the other “no-z” HzRG-candidates. It seems reasonable to assume that, on average, compact sources are younger than more extended
ones (Blundell, Rawlings & Willott 1999). The dense circumnuclear dust configuration
would then quench the Lyα emission.
This suggests a scenario in which massive radio galaxies form with a large starburst, perhaps simultaneously in several smaller, merging components. This starburst
manifests itself as a luminous “sub-mm source”. During the next stage a massive black
hole is formed and/or activated, but any rest-frame optical emission remains obscured
by dust from the starburst. As the radio source evolves it grows and expels the obscuring dust envelope, becoming visible as a HzRG (c.f. Jarvis et al. 2002; Reuland et al.
2003). This is essentially the radio-loud version of scenarios where “Ultra Luminous
Infrared Galaxies” evolve into quasars as envisaged by Sanders et al. (1988a).
Radio galaxies are the most massive and most extended galaxy sized systems that
are known to exist at high redshift. They are therefore good targets for studying the origin of the relationship between galaxies and their central black holes. WN J0305+3525
may be at the stage where both the galaxy and the black hole are being put together.
As with other sub-mm sources it is difficult to obtain spectroscopic redshifts and it
is important to obtain further information about their SEDs at mid- and far-IR wavelengths. X-ray imaging could show whether or not there is a bright AGN associated
with WN J0305+3525. We are actively pursuing follow-up observations to address
some of these issues.
Acknowledgments
We gratefully acknowledge the help of the Keck, and JCMT staff. We thank the IRAM
staff and Albrecht Sievers in particular for carrying out observations in Director’s Discretionary Time, project Delta 00-04, and reducing the data. The work of M.R., W.v.B.,
and W.d.V. was performed under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. The work of D.S. was
carried out at the Jet Propulsion Laboratory, California Institute of Technology, under
a contract with NASA.
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Chapter 4
Dust enshrouded compact radio-loud
AGN at high redshift
Michiel Reuland, Wil van Breugel, Wim de Vries, Huub Röttgering, and Carlos De Breuck,
Monthly Notices of the Royal Astronomical Society, submitted
We present submillimetre and X-ray observations of a sample of ten radio galaxies
for which no spectroscopic redshift could be determined in 1–2 hr Keck observa< 2 00 ) radio mortions. Generally, these galaxies are characterised by compact (θ ∼
1400MHz
< −1.30; Sν ∝ ν α ). The galaxphologies and ultra–steep radio spectra (α325MHz ∼
ies were selected from a radio sample with flux densities 10 < S 1.4GHz < 100 mJy,
fainter than most previous surveys. Four of the ten galaxies were detected in the
submillimetre with S/N > 4. Circumstantial evidence suggests that they are at
redshifts z > 3 and then constitute up to 30% of the high redshift radio galaxy pop> 19), were generally not
ulation. Their host galaxies are faint in the near-IR (K ∼
>
detected in the optical (R ∼ 24) or X-ray, but appear to be as bright as z > 3 radio galaxies at submillimetre wavelengths. This suggests that these galaxies are
embedded in dense gaseous environments, from which they are forming. The
high submillimetre detection fraction and the signal h S CSS
850,<3σ i = 2.48 ± 0.48 mJy
in the stacked non-detections indicate that this compact obscured phase may be a
common ingredient in AGN evolution. Based on the empirical radio to X-ray relation for AGN, intrinsic X-ray luminosities of L2−10 keV ∼ 1044−46 erg s−1 and column
densities of a few 1023 cm−2 are inferred, showing that they could maybe contribute
to the hard X-ray background.
1 Introduction
P
OWERFUL radio galaxies and quasars are among the most luminous galaxies and
can be studied in great detail out to large distances (e.g., De Breuck et al. 2002, and
references therein). Their radio structures, powered by actively accreting super massive black holes, interact strongly with the host galaxies and environments. Therefore,
they are excellent laboratories for studies of galaxy formation and black hole growth
in the early universe. The powerful radio source population comes in a wide variety
of sizes and ranges from the very compact Gigahertz Peaked Spectrum (GPS; < 1 kpc)
and Compact Steep Spectrum (CSS; 1–20 kpc) objects to the classical edge brightened
Fanaroff & Riley class II (FR II; up to > 1 Mpc; Fanaroff & Riley 1974) radio sources.
The consensus is that for low redshifts (z < 1) this is best interpreted as evidence for
47
48
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
evolution of radio source size with age (e.g., Phillips & Mutel 1982; de Vries 2003). The
situation at higher redshifts, however, is much less clear.
The average density and galaxy merger rate must have been much higher in the
early universe than they are presently. This is likely to have resulted in a stronger interaction between early radio sources and their environments. Conceivably, the ISM
could have been sufficiently dense to prevent a large fraction of GPS/CSS sources from
expanding (the frustration scenario; e.g., van Breugel et al. 1984). A difference in evolution between low and high redshift sources is suggested also by the fact that almost
all known GPS/CSS sources at redshifts z < 1 are associated with galaxies, whereas
for z > 1 they are associated with quasars. Their X-ray properties have been found
to be different as well (e.g., O’Dea et al. 2000; Siemiginowska et al. 2003). This is
unexpected in the simple picture that quasars and galaxies are alike, but viewed under different angles. The GPS/CSS sources constitute approximately 30–40% of bright
centimeter-frequency-selected samples (O’Dea 1998). Observations of such compact
radio galaxies at high redshift could further our understanding of galaxy formation
processes and jet triggering events.
Another incentive to study young active galactic nuclei (AGN) stems from deep Xray surveys (Chandra, XMM, ROSAT) which have shown that the cosmic X-ray background (XRB) is largely due to discrete, actively accreting supermassive black holes
(see Hasinger 2003,for a recent review). These consist of a mixture of unobscured (Type
I) and obscured (Type II) AGN. The obscured AGN may have space densities several
times larger than unobscured AGN and could contribute up to 40% of the 5–10 keV
cosmic X-ray background (e.g., Mushotzky et al. 2000; Giacconi et al. 2001). Because
of their obscuration, optical identifications have remained elusive and their properties
and ubiquity have been poorly explored, requiring more observations.
1400MHz
Spectroscopic followup of sources with an ultra steep radio spectrum (USS; α 325MHz
< −1.30; Sν ∝ ν α ) and faint near-IR counterpart show redshifts z > 3 in ∼35% of the
∼
cases (for details see De Breuck et al. 2001). However, a significant fraction (∼25%)
of the galaxies that fulfill all requirements of our efficient high redshift radio galaxy
(HzRG; z > 2) candidate selection criteria fail to yield emission line redshifts in 1–2 hr
spectroscopic observations at the 10 m Keck telescopes. Generally, these are compact
< 2–3 00 ). A possible explanation for the lack of line emission is that they are
objects (θ ∼
situated in extremely dense and dusty environments. One of them has been studied
before in detail (WN J0305+3525; Reuland et al. 2003) and was found to be very (sub)millimetre (submm) luminous and highly obscured.
Could it be that all these USS compact sources are heavily obscured? Does the
fraction of such sources increase with redshift? Is such an obscured and compact phase
a universal stage in the birth process of radio galaxies? This could hint at a radio-loud
analog to the ULIRG–QSO evolutionary scheme presented by Sanders et al. (1988a) and
Haas et al. (2003), where a luminous submm phase partly coincides with the emergence
of a HzRG. What are the intrinsic X-ray luminosities and obscuring column depths,
and could the AGN be part of the population that contributes to the hard XRB?
To help answer these questions we have obtained (sub-)mm (SCUBA/JCMT, MAMBO
/IRAM) and X-ray (Chandra) observations of 10 HzRG-candidates of this intriguing
class. Results of this observing campaign are reported in this paper.
Section 2. Observations and Results
49
Figure 1 — Contour overlays of 5 GHz VLA maps of WN J0305+3525 (left) and WN J2044+7044 (right)
on the 0.5–7 keV images obtained with Chandra. The contour levels are 0.5, 1.0, 2.0 mJy beam −1 for
WN J0305+3525 and 0.5, 1.0, 2.0, 3.0 mJy beam−1 for WN J2044+7044.
Source
WNJ0305+3525
WNJ0310+3644
TNJ1026−2116
WNJ1314+3649
WNJ1355+3848
WNJ1525+3010
WNJ1836+5210
WNJ1917+6635
TNJ1954−1207
WNJ2044+7044
RA(J2000)
hms
DEC (J2000)
03 05 47.42
03 10 54.80
10 26 22.37
13 14 17.86
13 55 29.50
15 25 01.21
18 36 23.22
19 17 35.50
19 54 24.15
20 44 57.80
+35 25 13.4
+36 44 02.5
−21 16 07.7
+36 49 14.6
+38 48 11.1
+30 10 30.2
+52 10 28.4
+66 35 38.5
−12 07 48.7
+70 44 03.8
◦0 00
NH
cm−2
LAS
1.2×1021
1.2×1021
6.6×1020
1.0×1020
9.4×1019
2.0×1020
4.5×1020
6.2×1020
1.0×1021
1.7×1021
1.9
2.0
1.0
1.3
2.3
1.2
1.4
1.9
< 7.0
1.3
00
.4
α1325
-1.33
-1.70
-1.39
-1.41
-1.42
-1.46
-1.41
-1.30
-1.38
-1.38
S1.4 GHz
mJy
K
mag
R1
mag
R−K
mag
Continuum
15.8
24.2
61.5
36.4
25.6
17.2
24.2
11.2
102.8
41.7
20.7
−
19.8
22.1
19.3
19.5
17.7
20.0
19.9
19.2
23.4*
23.0*
> 24
> 26
> 25
25.3
21.0
23.7
−
> 24
2.7*
−
> 4.2
> 3.9
> 5.7
5.8
3.3
3.7
−
> 4.8
U
F
U
F
F
U
B
F
F
U
Table 1 — Radio positions, Galactic H I column densities NH , largest angular radio sizes, radio spec.4 , 1.4 GHz flux densities, K and R-band magnitudes, R − K colours. The near-IR and
tral indices α1325
optical magnitudes were all measured within 4 00 diameter circular apetures, only for WN J0305+3525 a
6 00 diameter aperture was used due to the extended nature of the source. The spectra of some sources
showed continuum emission but no identifiable features. The last column indicates whether the continuum was bright (B; F
> 2µJy), faint (F; F
< 2µJy) at 5000 Å (close to the blueward limit of
5000 Å
5000 Å
the spectrographic observations for most sources) or whether it remained undetected (U). For the TN
sources the spectral index has been estimated using observations at 365 MHz as the lower frequency
instead of 325 MHz. These data were taken from the following papers: De Breuck et al. (2000, 2001); De
Breuck et al. (2002) and Reuland et al. (2003).
This paper is organised as follows. In §2 the target selection, details on the observations and observational results are presented. We discuss redshift estimates, possible
scenarios to explain the results and their implications in §3. These are then summarised
in §4. We assume a flat universe with H0 = 65 km s−1 Mpc−1 , ΩM = 0.3 and ΩΛ = 0.7
throughout.
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
50
Source
WNJ0305+3525
WNJ0310+3644
TNJ1026−2116
WNJ1314+3649
WNJ1355+3848
WNJ1525+3010
WNJ1836+5210
WNJ1917+6635
TNJ1954−1207
WNJ2044+7044
S850 a
mJy
12.5 ± 1.5
7.0 ± 1.6
0.2 ± 1.7
5.3 ± 1.2b
2.9 ± 1.3
3.5 ± 1.6
2.8 ± 1.3
1.1 ± 1.4
3.6 ± 1.3
5.6 ± 1.3
S/N
8.3
4.2
0.1
4.3
2.3
2.1
2.2
0.8
2.8
4.3
3σ limit
mJy
< 5.14
< 6.68
< 8.37
< 6.58
< 5.27
< 7.44
S1250
mJy
4.2 ± 0.6
−
−
2.2 ± 0.7
−
−
−
−
−
1.4 ± 0.6
S/N
K
zest
7.4
−
−
3.3
−
−
−
−
−
2.3
>3
−
>1
>5
>2
> 1.5
> 0.5
>2
> 1.5
>2
Table 2 — Observed 850 µm and 1250 µm flux densities with their standard errors, and redshift estimates based on the K-band magnitudes of the radio sources in the program. 3σ upper limits are shown
for sources whose S/N ratios are below 3. The lower limits to the redshift estimates are based on the
faint envelope (1.5 mag fainter than the best-fit average) of the K–z relation for HzRGs (Fig. 7 in De
Breuck et al. 2002).
a
This does not include the 10–15 per cent uncertainty in absolute photometric calibration.
b
Affected by an estimated 4 00 pointing error due to problems with the tracking model for observations with elevations above 60 degrees. See http://www.jach.hawaii.edu/JCMT/Facility description/Pointing/tracking fault.html for details.
2 Observations and Results
The targets were selected from a sample of the 669 USS radio sources compiled by De
Breuck et al. (2000). This sample has been constructed using radio flux densities that
are 1–2 orders of magnitude fainter than steep spectrum samples based on previous
radio catalogs such as 3C, 4C, and 6C. It forms the basis of our ongoing search for
HzRGs. De Breuck et al. (2001) found that ∼25% of the galaxies that fulfill all requirements of our HzRG-candidate pre-selection criteria (see De Breuck et al. for details)
failed to yield emission line redshifts in 1–2 hrs optical and near-IR spectrographic observations with LRIS or NIRSPEC at the Keck 10 m telescope. Some of these did not
> 24–25) and only have faint near-IR (K ∼
> 19) couneven show optical continuum (R ∼
00
<
terparts. These “no-z” objects with a very compact (θ ∼2–3 ) radio structure are the
subject of the present study. The coordinates, references, R and K-band magnitudes
and relevant radio properties of our targets are given in Table 1.
2.1 SCUBA photometry
Observations were carried out between December 2000 and February 2001 with the
Submillimetre Common–User Bolometer Array (SCUBA; Holland et al. 1998) at the
15 m James Clerk Maxwell Telescope (JCMT). We observed at 850 µm wavelengths
resulting in a beam size of 15 00 . We employed the 9-point jiggle photometry mode, with
a chop throw of 45 00 in azimuth at 7.8 Hz. Frequent pointing checks were performed to
ensure pointings better than 2 00 .
The observing conditions were generally good with optical depths τ850 varying between 0.15 and 0.35. The data were reduced using the Scuba User Reduction Facility
Section 2. Observations and Results
Date
JD
texp
ks
Flux
counts
Backgr.
counts
3σ limit
counts
Net count rate
counts s−1
S0.5−10keV
erg s−1 cm−2
S2keV
mJy
αSX
Dec/01/2002
Jul/28/2003
Apr/14/2003
12.6
9.0
5.1
4
0
2
0.14
0.10
0.04
< 14.3
< 6.5
< 10.8
< 1.1 × 10−3
< 0.7 × 10−3
< 2.0 × 10−3
< 1.0 × 10−14
< 0.5 × 10−14
< 2.0 × 10−14
< 7.1 × 10−7
< 3.6 × 10−7
< 14.2 × 10−7
< −1.18
< −1.16
< −1.08
Source
WN J0305+3525
WN J1314+3649
WN J2044+7044
51
Table 3 — Summary of the Chandra observations, with 3σ Poissonian upper limits following Gehrels
(1986). The source and background count rates have been measured within 3.5 00 diameter apertures
and converted into X-ray flux densities assuming a photon index Γ = 2 and correcting for Galactic
absorption. The last column shows 3σ upperlimits to the submm–to–X-ray spectral index α SX and is
discussed in Section 3.2.2
software package (SURF; Jenness & Lightfoot 2000) following the procedure outlined
in Reuland et al. (2004). Flux calibration was performed using HLTAU, OH231.8 and
CRL618 as photometric calibrators. The typical photometric uncertainty for our program is of order 10–15 per cent (see e.g., Jenness et al. 2001).
The results of the SCUBA observations are summarised in Table 2. Of the ten
sources observed at 850 µm four were detected with flux densities S 850 > 5 mJy at
larger than 4σ confidence level.
2.2 IRAM Photometry
Following their detections with SCUBA, we observed WN J1314+3649 and WN J2044+
7220 at 1.25 mm using the MPIfR bolometer arrays (MAMBO II; Kreysa et al. 1998) at
the IRAM 30 m telescope in service mode. The 117-channel bolometer array was used
in December 2001 and the 37-channel bolometer array during January and February
2002. The symmetric “ON-OFF” mode with wobbler throws in the range of 32–46 00 was
employed and the observations were done using on-offs with 16–20 subscans of 12 s
per subscan. The atmospheric extinction τ1.25 varied between 0.09 and 0.24. Pointing
corrections were found to be better than 2 00 . The on-off data was reduced with MOPSIC
with sky-noise filtering2 .
Including previous observations of WN J0305+3525, two out of three submm sources
have been detected (> 3σ ) at 1.25 mm with MAMBO (Table 2), confirming the reality
of the emission found with SCUBA.
2.3 Chandra
Three of the galaxies detected in the submm were observed with the Chandra X-ray
Observatory (Weisskopf et al. 2000) between December 2002 and July 2003. The exposures were obtained using the back-illuminated ACIS–S3 chip in TE mode with 3.2 s
readout and FAINT telemetry format. The data were processed using the standard tool
CIAO 2.33 . The detector lightcurves showed no evidence for contamination by flares,
therefore the total length of the exposures was used effectively. Following Giacconi
et al. (2001) we limited the bandpass to 0.5–7 keV, because inclusion of the 7–10 keV
band always decreases the signal-to-noise ratio. For comparison with other papers,
fluxes are quoted in the 0.5–10 keV band, as extrapolated from the measured count
2
3
Zylka, R. The MOPSI Cookbook
http://cxc.harvard.edu/ciao
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
52
rate. All count rates and X-ray flux densities are calculated using PIMMS v3.4 4 , assuming a photon index Γ = 2 which is appropriate for obscured AGN (Fabian et al.
2000).
Table 3 summarizes the Chandra observations. The X-ray counts were measured
within 3.5 00 diameter circular apertures, comparable to the extents of the radio sources.
The background was estimated using a large annular region, centered on the radio position, after removal of point sources. None of the sources observed with Chandra were
formally detected in the X-ray regime. Upper limits on the counts are estimated with
Poisson statistics following Gehrels (1986). Figure 1 shows that, despite the low number of counts, for WN J0305+3525 and WN J2044+7044 there apppears to be concentration of photons at the position of the radio emission. Especially for WN J0305+3525 it
is worth noting that in the 10 arcmin2 field for which we have I-band imaging the five
other regions that contain more than three photons in a 3.5 00 diameter aperture all have
obvious I-band counterparts. Hence it seems probable that the four photons detected
are indeed associated with WN J0305+3525.
3 Discussion
While we have detected a significant fraction of these “no-z” radio sources in the mm
and submm regime, none of the sources was detected significantly with Chandra.
These results prompt the following questions: What is the origin of the submm emission? Does the lack of X-ray photons indicate high obscuration by a dense medium
favouring the frustrated radio source scenario or is it because the targets are intrinsically weak X-ray emitters? Are these nearby starbursting systems or highly obscured
type II AGN possibly at high redshift? What are the H I absorbing column densities and how compare these with dust masses derived from the submm luminosities?
Where do these sources fit into radio galaxy evolution and AGN unification scenarios?
3.1 Redshift estimates
With the available data, it is impossible to obtain accurate redshift estimates. Still, we
can address whether or not the sources are at large cosmological distances. As discussed in Reuland et al. (2003) we have several indirect means of estimating their redshifts. Circumstantial evidence that a large fraction of these objects could be at redshifts
z ∼ 3 comes from the following: (i) They are USS selected radio sources which tend to
be at high redshifts (De Breuck et al. 2001), (ii) It has been long known that powerful radio galaxies trace the bright outer envelopes in so-called K-z diagrams which relate the
observed K-band magnitude (rest-frame UV/optical) to redshift (e.g., De Breuck et al.
2002). Recently such a relation was found to hold also for submm sources (Dunlop
2002), although a detailed comparison between the two populations by Serjeant et al.
> 2 mag fainter with a larger disper(2003) shows that the radio-quiet objects may be ∼
sion. Therefore, the faint K-band magnitudes suggest that they are at high redshift
K
(see column labeled zest
in Table 2). This is corroborated by their red near-IR colours
of R − K > 4–6 (see Table 1) typical of distant massive galaxies (McCarthy 2004), (iii)
4
http://heasarc.gfsc.nasa.gov/Tools/w3pimms.html
Section 3. Discussion
Source
WN J0305+3525
WN J1314+3649
WN J2044+7044
53
S325 MHz
mJy
110
286
311
S1.4 GHz
mJy
15.8
36.4
41.7
S4.8 GHz
mJy
3.74
−
7.18
.4
α1325
α41..84
-1.33
-1.41
-1.38
-1.17
−
-1.43
Table 4 — Summary of radio flux densities and spectral indices of the sources observed with Chandra
Their submm detection fraction of four out of ten is closer to the one for z > 2.5 HzRGs
in surveys of comparable sensitivity (∼50%; Archibald et al. 2001; Reuland et al. 2004)
than to the fraction (∼15%) for lower redshift radio galaxies. (iv) For six objects no
emission is found blueward of 4800 Å, which corresponds to the Lyα break at z ∼ 3.
In contrast, WN J0310+3644 and WN J1355+3848 show very faint continuum emission
possibly down to 4000 Å, and for WN J1836+5210, and TN J1954−1207 the spectra cut
off at 4800 Å where the continuum is still fairly bright. Clearly, those four sources must
be at z < 3 although z ∼ 2 is still possible.
While none of these indicators by itself can be considered compelling, for most
cases they favour redshifts z ∼ 3. In the following we assume a redshift z = 3 to be
representative for the sources and discuss possible implications.
3.2 X-ray properties
Since none of the objects were formally detected with Chandra, we compare the upper
limits with expected count rates using various assumptions discussed below.
3.2.1 Radio–X-ray relation
There is a plethora of X-ray emission processes operating in radio galaxies such as
Inverse Compton scattering of cosmic microwave background photons or local far-IR
radiation fields, Synchrotron Self Compton radiation, synchrotron emission from the
lobes, jets or cores, and thermal emission from shocked gas. Brinkmann et al. (2000)
investigated radio–X-ray (Lr / LX ) correlations for a variety of objects (both radio-loud
and radio-quiet) from a comparison between the ROSAT All-Sky Survey and the VLA
1.4 GHz FIRST catalogue. The processes responsible for radio-related non-thermal nuclear X-ray emission and the characteristic scales on which they operate are not well
understood but the simplest model may be one in which the X-rays originate in the jet
itself (for a discussion see Hardcastle & Worrall 1999).
The Brinkmann et al. relation for flat spectrum radio-loud AGN to estimate the
expected unobscured X-ray luminosity LX at 2 keV given a 5 GHz radio luminosity Lr
is:
log LX = (11.2 ± 1.6) + (0.48 ± 0.05) × log Lr ,
(1)
with Lr and LX in erg s−1 Hz−1 .
Table 4 lists radio properties from the literature (De Breuck et al. 2000) of the three
sources observed with Chandra. The inferred rest-frame radio and X-ray luminosities
are given in Table 5. We used PIMMs to convert these luminosities into count rates if
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
54
Source
zest
WN J0305+3525
WN J1314+3649
WN J2044+7044
3.0
3.0
3.0
log(L325 MHz )
W Hz−1
28.2
28.7
28.7
log(L1.4 GHz )
W Hz−1
27.3
27.8
27.8
log(L5 GHz )
W Hz−1
26.6
27.0
27.0
log(L2 keV )
erg s−1 Hz−1
27.45
27.64
27.64
log(L0.5−2 keV )
erg s−1
45.27
45.46
45.46
log(L2−10 keV )
erg s−1
45.34
45.53
45.53
Table 5 — Inferred radio and unobscured X-ray luminosities of the sample. The X-ray luminosities are
estimated from the 5 GHz radio luminosity L5 GHz using equation 1 and assuming a photon index Γ = 2.
Source
zest
WN J0305+3525
WN J1314+3649
WN J2044+7044
3.0
3.0
3.0
log(L0.5−10 keV )
erg s−1
45.61
45.80
45.80
S0.5−10keV
erg s−1 cm−2
4.54 ×10−14
7.03 ×10−14
7.03 ×10−14
S0.5−10 keV
counts s−1
5.0 ×10−3
9.8 ×10−3
7.1 ×10−3
S2−10 keV
counts s−1
1.1 ×10−3
1.8 ×10−3
1.8 ×10−3
Table 6 — Predicted unobscured X–ray count rates for Chandra. These count rates are based on the
X–ray luminosites as derived using equation 1 and assuming a typical photon index Γ = 2. The count
rates are corrected for the Galactic foreground columns NH given in Table 1.
they would be observed with the Chandra ACIS-S chips, as shown in Table 6. Comparison of these predicted count rates (∼ 1 × 10−2 counts s−1 ) with the observations show
< 1 × 10−3 counts s−1 ; Table
that they are an order of magnitude higher than measured (∼
3). Below we discuss possible explanations for this discrepancy.
One explanation might be that the trend described by equation (1) does not hold
for our USS sources. Unfortunately, no Lr / LX correlations exist in the literature for
USS galaxies. A complication with predicting the intrinsic X-ray luminosities of the
objects in the present study from their radio emission is that the compact USS sources
are likely to be dominated by the lobe emission (for nearby CSS sources < 0.4% of
the radio luminosity originates from the core; Fanti et al. 1995) since they are almost
unresolved and intrinsically small. However, Worrall et al. (2004) showed that the lobedominated (< 1% of the 1.5 GHz flux arises from the core) CSS source 3C48 falls on the
relation for flat-spectrum sources, suggesting that the relation remains applicable.
3.2.2 Submm–X-ray relation
One could explain the lack of X–ray emission if the radio emission were due to nonAGN related activities. Star formation seems a viable alternative as it can lead to significant radio emission (Lfar−IR /Lradio relation; Condon 1992) and (as was demonstrated
for WN J0305+3525) the observed submillimetre emission is likely related to vigorous starbursts. For comparison we take the ultra luminous infrared starburst galaxy
Arp 220 as a template. Arp 220 has a 1.4 GHz radio luminosity of log L 1.4GHz = 23.41.
Assuming similar radio luminosities for our sources would imply redshifts on the order of z ∼0.05–0.08. Such low redshifts, however, would be at odds with the K > 19
magnitudes and observed submillimetre flux densities of a few mJy because Arp 220,
which is at z = 0.02, is orders of magnitudes brighter with K = 9.852 ± 0.036 (Jarrett
et al. 2003) and S850 = 832 ± 86 mJy (Dunne et al. 2000). This shows that almost the
entire radio emission for our sources must be related to an AGN.
The submm-to-X-ray energy spectral index αSX , is another tool to differentiate be-
Section 3. Discussion
55
Figure 2 — Submillimetre–to–X-ray energy
The
spectral index, αSX , versus redshift.
horizontal line indicates the 3σ upper limit
to αSX for the three sources in the program.
The expected αSX values for Compton thin
AGN (3C 273), obscured AGN (NGC 6240;
assuming f sc = 0.05), and ultraluminous
starburst galaxies (Arp 220) are overplotted.
Alternate curves for NGC 6240 with a smaller
scattered flux fraction ( f sc = 0.01) and less
internal absorption (NH = 5 × 1023 cm−2 ) are
plotted also. This figure has been adapted from
Fabian et al. (2000).
tween Compton-thin AGN, Compton-thick AGN, and starburst systems (see Figure 2
and Fabian et al. 2000). This index is only a weak function of redshift, and therefore es< −1.1 (see
pecially suited to analyse the “no-z” sources. The spectral indices of order ∼
Table 3), are inconsistent with Compton-thin AGN (such as 3C 273) at any redshift and
confirm that the sources most likely host highly obscured AGN although pure starbursts cannot be ruled out based on this parameter alone. The double radio structure
of WN J2044+7044 together with the difficulty of attributing the radio emission purely
to star formation strongly favours highly obscured nuclear activity to simultaneously
explain the radio emission and paucity of X-ray photons.
3.2.3 Obscured nucleus
That the USS sources appear underluminous in the X-rays is not all that surprising. In
orientation based unification schemes of AGN (e.g., Barthel 1989) emission from the
cores of radio galaxies passes through an obscuring torus. This torus is optically thick
in the optical and, depending on its properties, may even strongly absorb hard X-rays
(0.5–10 keV observed frame corresponds with 2–40 keV rest-frame at a redshift z = 3).
Since the relation used above was derived for flat spectrum quasars which are being
observed close to the jet axis allowing a direct view of the core emission, the obscuring
effects of this torus must be taken into account.
In Table 7 we list the predicted count rates for the same luminosities as in Table 6, but include an additional obscuring medium with column densities NH of 1–
100×1022 cm−2 at the redshift of the source. From this table we infer that column densities NH of a few ×1023 cm−2 are sufficient to explain the fact that we did not detect
the sources. These values are slightly higher than but consistent with the findings by
O’Dea (1998), who inferred absorbing columns of at least a few ×1022 cm−2 to explain
why GPS/CSS galaxies are approximately two orders of magnitude fainter in the Xrays than quasars. Similar absorption columns were found also by Siemiginowska
et al. (2003).
56
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
Powerful FR II radio galaxies are also known to be observed through such obscuring tori, with most observed X-ray and radio emission arising from a diverse variety of
processes in the extended components. Comparison with the intrinsic X-ray luminosities of these sources could serve as an important check, however the paucity of X-ray
data for HzRGs makes direct comparisons complicated. Carilli et al. (2002) report that
for Cygnus A and MRC 1138−262 the ratio between unobscured nuclear X-ray emistotal
sion and total radio power Lnucleus
2−10keV / L0.1−10GHz ∼ 2.5 is constant to within a factor of
two. Scaling with the 0.1-10 GHz radio power yields hard X-ray luminosities L 2−10keV
of order ∼ 5 × 1044 erg s−1 for our sources, similar to but slightly below the values derived using equation 1 (see Table 5). Overzier et al. (2004) also report that 3 out of 4
sources in their sample of z ∼ 2 radio galaxies agree well with the L r / L X relation if they
use a photon index Γ = 1.8 to calculate the luminosity at rest-frame 2 keV. It must be
noted however, that a much lower value for the nuclear X-ray emission is found by
Scharf et al. (2003) for the archetype HzRG 4C 41.17 at z = 3.8. They report an intrinsic
X-ray luminosity of only 9 × 1043 erg s−1 instead of 1 × 1045 erg s−1 as expected based
38
on a radio power of Ltotal
0.1−10GHz 3 × 10 W. If the X-ray luminosities of the compact USS
sources would fall below the Lr /L X relation by the same amount as 4C 41.17, intrinsic
column depths NH ∼ 1022 cm−2 would be sufficient to explain why we did not detect
their X-ray emission.
3.3 Young starbursting radio galaxies
Based on the circumstantial evidence discussed above, it seems likely that many of
the compact USS sources in this program are indeed at large cosmological distances.
Since all the submm emission must be related to dust emission (extrapolation of the
USS radio spectrum according to a power law shows the synchtron contribution at
submm wavelengths to be negligble and heating of dust by AGN does usually not
contribute significantly at rest-frame far-IR wavenlengths; e.g., Archibald et al. 2001;
Farrah et al. 2002), this implies large star formation rates ∼ 1500 M yr−1 and dust
masses Md ∼ 1.5 × 108 M (see e.g., Reuland et al. 2003).
The lack of line emission, the high obscuration inferred from the X-ray emission,
and their compact sizes all point towards a scenario in which these galaxies are situated
in dense gaseous environments, possibly signatures of merger-induced starbursts, that
are both fueling the central engine and forming the galaxy.
Indeed, the medium that is obscuring the X-ray emission could be related to the
starburst. If one assumes intrinsic column densities of NH ∼ 1023 cm−2 at a radius
of ∼ 5 kpc (since it must be larger than the emission line region or else we should
have detected it in the optical spectroscopy) then one derives a total H I mass of order
2 × 1011 M . For typical ratios (as appropriate for ULIRGs; Sanders et al. 1991) of
MHI / MH2 ∼ 2 and MH2 / Md ∼ 500 this yields dust masses Md ∼ 2 × 108 M , consistent
with the submm emission.
This would suggest that the obscuration may be related to a young evolutionary
stage in a radio-loud analog to the ULIRG-QSO sequence first presented by Sanders
et al. (1988a) and phase 1-2 in the more detailed scheme of Haas et al. (2003). If the
submm emission would be directly related to the jet-triggering event we would have
expected a submm detection fraction higher for these compact objects than for regular
Section 3. Discussion
57
Source
zest
WN J0305+3525
WN J1314+3649
WN J2044+7044
3.0
3.0
3.0
log(L0.5−10 keV )
erg s−1
45.61
45.80
45.80
log(NH ) = 22
counts s−1
4.7 ×10−3
9.1 ×10−3
6.7 ×10−3
log(NH ) = 23
counts s−1
3.3 ×10−3
5.7 ×10−3
4.8 ×10−3
log(NH ) = 24
counts s−1
0.9 ×10−3
1.4 ×10−3
1.3 ×10−3
Table 7 — Predicted 0.5–10 keV count rates for Chandra. These count rates are similar to the ones in
Table 6, but assume additional intrinsic obscuring columns of NH 1 × 1022 cm−2 , NH = 1 × 1023 cm−2 ,
and NH = 1 × 1024 cm−2 at the redshift of the source.
HzRGs. Interestingly, this may be what we have observed, since the inverse variance
weighted average of the non-detections h SCSS
850,<3σ i = 2.48 ± 0.48 mJy is higher than the
HzRGs
weighted average h S850,<3σ i = 1.39 ± 0.30 mJy for 17 regular radio galaxies at z > 2.5
(c.f. Archibald et al. 2001; Reuland et al. 2004). Such a relation between compactness
and submm emission was not found for other radio galaxies, but has been reported for
radio-loud quasars at z ∼ 1.5 (Willott et al. 2002) for which it was attributed to a small
secondary nuclear starburst or quasar heated dust.
Comparison with HzRGs shows that the typical K-band magnitudes of the compact
sources are ∼0.5–1 mag fainter. If they are at comparable redshifts, then an explanation
may be the high obscuration (some HzRGs may suffer significant line contamination
in the K-band; Eales & Rawlings 1993), or that they are in an earlier stage of their
evolution with a lower fraction of their stellar masses having formed.
3.4 Implications for Type II AGN and XRB
Since none of the galaxies was detected in the X-rays it is difficult to establish their
contribution to the hard X-ray background. The upper limits on the count rates are
consistent with them being quite similar to (and possibly even more luminous than)
the type II QSOs detected in much deeper observations (see Table 8 and Stern et al.
2002; Norman et al. 2002). The similarity goes further, since their K-band magnitudes
are rather faint for regular HzRGs but more similar to the z > 3 type II QSOs, and
one type II QSO at z = 3.7 has now been detected in the submm with a comparable
flux density (S850 = 4.8 ± 1.1 mJy; Mainieri et al. 2004). A problem is that the space
density of such CSS sources is uncertain. While traditionally it is thought that only
10% of the QSOs is radio-loud and therefore less interesting statistically, it is worth
noting that recent surveys indicate that the bimodality in the radio-loud versus radioquiet distribution may be much weaker (White et al. 2000). This is consistent with
suggestions that radio loudness may increase with the mass of quasar host galaxies
and their central black holes (McLure et al. 1999; Dunlop et al. 2003) or, perhaps more
fundamentally, with accretion rate (Ho & Peng 2001). Furthermore, it seems possible
that many high redshift CSS sources are going through a dusty submm luminous obscured optical/X-ray phase. Especially for low power sources their radio-jets could be
contained by the large amounts of dust such that they do not always evolve into the
large radio structures typical of HzRGs. This could imply that the ratio of obscured
to unobscured AGN could have been higher at high redshift. Until recently there was
little overlap (5–10%) between X-ray and submm samples. However, Smail et al. (2003)
Chapter 4. Obscured compact ultra steep spectrum radio galaxies
58
Source
B2 0902+34
4C 41.17total
4C 41.17core
MRC 1138core
CXO 52
CXOUJ 2153+1742
CXOCDFS 0332-2751
WN J0305+3525
WN J1314+3649
WN J2044+7044
Type
z
HzRG
HzRG
3.395
3.798
HzRG
QSO II
QSO II
QSO II
2.156
3.288
0.7–1
3.700
3:
3:
3:
S1.4GHz
mJy
329
264
.4
α1325
−0.91
−1.15
600
<1
−1.20
15.8
36.4
41.7
-1.33
-1.41
-1.38
N
counts
94
162
37
663
54
86
130
< 14.3
< 6.5
< 10.8
Count rate
counts ks−1
9.7
1.2
0.3
19.4
0.29
9.5
0.13
< 1.1
< 0.7
< 2.0
Reference
Fabian, Crawford, & Iwasawa (2002)
Scharf et al. (2003)
Carilli et al. (2002)
Stern et al. (2002)
Fabian et al. (2000)
Norman et al. (2002)
Table 8 — Comparison of photon fluxes (N) and count rates of the sources in this programs with Chandra observations of HzRGs and type II QSOs from the literature.
found statistical evidence for X-ray counterparts to submm sources in overdense fields
around HzRGs for a similar fraction (∼40%) as found in the deep 2 Ms Chandra observation discussed by Alexander et al. (2003). This indicates that we do expect to find
Chandra counterparts to our bright submm sources. No systematic searches for these
objects have been conducted to date, but the implications for both AGN evolutionary
scenarios and the XRB could be significant.
Bauer et al. (2004) analysed X-ray number counts in the 1–2 Ms Chandra Deep Fields
to determine the properties of the populations contributing to the XRB. They found
that the soft XRB is dominated by relatively unobscured (NH < 1022 cm−2 ) luminous
L0.5−8 keV > 1043.5 erg s−1 sources. The hard XRB seems dominated by intrisically less
luminous sources with L0.5−8 keV = 1042.5 − 1044.5 erg s−1 and a broad range of absorbing column densities NH ∼ 1022 − 1024 cm−2 . They interpreted this trend as evidence
that even less intrinsically luminous, more highly obscured AGN may dominate the
number counts at higher energies where the XRB intensity peaks (20–40 keV). It seems
reasonable to postulate that the compact USS sources discussed in this paper and their
lower luminosity counterparts could be part of these latter two populations.
4 Summary
We have presented SCUBA, MAMBO and Chandra observations of ten compact ultra
steep spectrum radio sources for which no spectroscopic redshift could be determined
and compared those with earlier results of HzRGs and type II QSOs.
Four of the objects were detected in the submm, very similar to the detection rate
of z > 2.5 radio galaxies. The strong statistical submm signal from the non-detections,
their faint K > 19 magnitudes, and the lack of line emission suggest that a significant
fraction (30%) of USS sources may be very obscured HzRGs, possibly in an early stage
of their evolution. More observations are necessary to establish whether this obscured
phase is a common ingredient in AGN evolution.
As with other distant submm sources it is difficult to obtain spectroscopic redshifts
for such targets. Therefore, it is important to obtain further information about their
spectral energy distributions at mid- and far-infrared wavelengths with Spitzer. Even
then continuum far-IR/submm photometric redshifts may not be very accurate, unless
an estimate for the luminosity-dust temperature for radio galaxies can be determined
Section 4. Summary
59
(Blain et al. 2003). Searching for PAH features with IRS spectroscopy seems the best
strategy to determine their redshifts.
Despite the lack of X-ray photons detected, we have inferred large intrinsic X-ray
luminosities and obscuring column densities based on the emperical L r / LX relation for
AGN. If this obscured CSS radio phase is common, then these sources could contribute
to the hard XRB. Much deeper observations would be required however, to obtain solid
X-ray detections and measure their photon indices.
Acknowledgments
We thank Robert Zylka and Andrew Baker for their help and suggestions on the MOPSIC data reduction. We gratefully acknowledge the help of the staff at the JCMT and
IRAM observatories. In particular the help of Remo Tilanus was indispensable. We
thank Roderik Overzier, Bram Venemans and Andrew Zirm for productive discussions. The JCMT is operated by JAC, Hilo, on behalf of the parent organizations of
the Particle Physics and Astronomy Research Council in the UK, the National Research Council in Canada and the Scientific Research Organization of the Netherlands.
IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The
work of M.R., W.v.B., and W.d.V. was performed under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of
California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng48. W.v.B. also acknowledges NASA grant CXO-05700580 in support of HzRG research
with Chandra at LLNL.
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Chapter 5
The influence of ISM characteristics and
AGN activity on the far infrared spectral
energy distributions of starburst galaxies
We examine the far infrared spectral energy distributions (SEDs) of 41 local ultraluminous infrared galaxies (ULIRGs) using data from the literature. These SEDs are
fitted with theoretical SEDs for dust enshrouded solar metalicity starbursts presented by Dopita et al. (2004) that have been expanded to include a dusty narrow
line region. From these models it is found that almost all ULIRGs are best fitted
with high (P/k > 106 cm−3 K) pressures of the interstellar medium, similar to the
pressures inferred from emission line ratios. These high pressures are likely related
to the pressure above which blowout occurs in a galactic superwind. Furthermore,
infrared luminosity ( LFIR ) and the timescale over which molecular clouds dissipate appear correlated, indicating that bigger starbursts tend to be more dust enshrouded.
We have investigated the implications of our findings for high redshift sources. It
seems that the physical mechanisms may be similar to local ULIRGs, and that the
models are applicable also in the distant universe. Fitting of three high redshift
radio galaxies with good far-IR observations indicates that to fit their SEDs may
require even higher pressures than found for ULIRGs. Such high pressures could
be related to the presence of the radio source and possibly jet induced star formation. They would increase the effective temperature of these highly luminous
sources and agree with the infrared luminosity-temperature relation. Many previous studies have reported that the fraction of LFIR that is contributed by an active
galactic nucleus (AGN) increases with LFIR . We do not confirm such a correlation.
Rather, we find that the relative contribution of an embedded AGN can vary significantly for any LFIR . Taking this AGN related component into account can lower
star formation rates inferred from LFIR by a factor of 2–3. For the high redshift radio galaxies, the high dust temperatures together with hidden AGN activity would
lower the star formation rates to values lower than commonly inferred.
1 Introduction
O
NE of the primary goals of cosmology is to understand the processes controlling
mass assembly in the universe. Madau et al. (1996) used the Hubble Deep Field
to construct an overview of the observed star formation rate per unit comoving vol-
63
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
64
energy distributions of starburst galaxies
ume in the universe from the present to when it was less than 10% of its current age.
The initial overview was predominantly based on optical data. The brightness of the
cosmic infrared background (CIB) indicates that approximately half of the activity in
the universe is or has been obscured by dust (e.g., Puget et al. 1996; Fixsen et al. 1998;
Hauser & Dwek 2001). Therefore, large correction factors to the optically based star
formation rates are required.
In luminous infrared galaxies (LIRGs; LIR > 1011 L ; IR; 5–500 µm) most of the ultraviolet/optical light from their starbursts is absorbed by dust and re-emitted in the
infrared waveband (for a review see Sanders & Mirabel 1996). These LIRGs dominate the population of extragalactic objects in the local universe (z < 0.3). At even
higher IR luminosities lie the ultraluminous (ULIRGs; LIR > 1012 L ) and hyperluminous (HyLIRGs; LIR > 1013 L ) infrared galaxies. Although these are rare in the local
universe (∼0.1% of the local galaxy population; Kim & Sanders 1998), their comoving number density shows strong evolution with redshift (e.g., Lagache et al. 2003,and
references therein). It seems likely that the submillimeter galaxies (SMGs) are high redshift analogues to the local ULIRGs, with possibly ∼1000 times higher comoving space
densities (e.g., Scott et al. 2002). These high number densities and IR luminosities indicate that it is essential to probe obscured sites of star formation in order to understand
when and how the bulk of stars formed in the early universe.
The excellent performance of the Spitzer space observatory (Werner et al. 2004) has
spawned renewed interest in studies of the star formation history of the universe. It
is delivering data of significantly better quality in the 3–180 µm range than previous
missions or ground-based facilities, providing a detailed sampling of the IR part of
the spectral energy distributions (SEDs) of starburst galaxies in the local and distant
universe. Therefore, it is now timely to aim for a better understanding of physical
parameters controlling the SEDs of sources that are hosting massive starbursts, and
estimate their star formation rates.
Presently, many properties of ULIRGs are still unknown. One of the major unanswered questions concerns the longstanding starburst-AGN controversy. Is the dominant power source of ULIRGs, and by implication of the SMG population, AGN or
starburst related? Clearly, the answer to this question is crucial for our understanding
of mass assembly.
Dopita et al. (2004) have developed theoretical models for obscured starbursts. The
models combine state-of-the-art radiative transfer for evolving H II regions with a sophisticated treatment of embedded dust. In that paper (the first of a series) the dependence of the SEDs on the pressure of the interstellar medium and the molecular cloud
dissipation time scale was investigated and a first comparison with the observed SEDs
of Arp 220 and NGC 6240 was made.
In the present paper we have expanded the Dopita et al. starburst models to include a model dusty narrow line region. With these models we cannot only derive star
formation rates for obscured sources, but also probe the underlying physical parameters of the ISM. Moreover, since we are modeling the entire SEDs and have included
an obscured AGN we obtain a handle on the dependence of the SEDs on the power
source. This allows us to estimate the fractional contribution of AGN activity to far-IR
luminosities LFIR commonly used to infer star formation rates for distant objects such
Section 2. The Samples
44.0
42.0
4
logνFν (erg s -1)
43.0
10 6
k=
10 7
P/
0
=1
P/k
Figure 1 — The starburst spectral energy distributions for ISM pressures
P/k = 104 , 106 and 107 cm−3 K, scaled
to an absolute star formation rate of
1 M yr−1 and computed for the case
in which the molecular cloud covering factor decreases linearly with time.
This figure is reproduced from Dopita
et al. (2004) in which a detailed description of the underlying assumptions and
model parameters can be found. For
a linearly decreasing molecular cloud
dissipation scale, pressures of P/k =
104 , 106 , 107 cm−3 K correspond to single temperature modified blackbody
dust temperatures of Tdust = 30, 35, and
41 K respectively.
65
41.0
40.0
39.0
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
log λ (µm)
as SMGs.
This study has two main goals. First, to check the applicability of the Dopita et al.
models using literature data for a large sample of local ULIRGs, estimate the fractional
AGN contributions to LFIR , investigate the pressures in the ISM, and obtain estimates
for the star formation rates. Secondly, to extend this investigation to a few high redshift
sources with good far-IR observations.
The structure of this paper is as follows: The sample selection is described in §2. In
§3 we present some key aspects of the models and fitting procedures used. §4 presents
the results. The implications for starburst galaxies at low and high redshift are discussed in §5, and in §6 we provide a summary of our conclusions.In this paper we
assume a flat universe with ΩM = 0.27, ΩΛ = 0.73, and H0 = 71 km s−1 Mpc−1 (Spergel
et al. 2003; Tonry et al. 2003).
2 The Samples
For comparison with the theoretical models we have compiled data for low and high
redshift starbursts from the literature. Below we first describe the sample selected and
data acquisition for a detailed analysis of ULIRGs at low redshift. Then we discuss
the selection of high redshift sources for which good sampling of their SEDs is available. These allow us to investigate how the methods developed in this paper apply to
starbursts in the early universe.
2.1 ULIRGs
We have selected the 41 ULIRGs observed with ISO by Klaas et al. (2001), because
their SEDs are well sampled over the wavelength regime 1–200 µm. This sample is
representative of nearby (z < 0.2) ULIRGs, but also includes the slightly less luminous
objects NGC 6240, Mrk463, and four objects with redshifts in the range 0.2 < z < 0.4.
Flux densities were collected using the NASA/IPAC Extragalactic Database (NED)
supplemented with a wide selection of on-line catalogues and papers. Generally, the
UV/optical fluxes were taken from the third reference catalogue of bright galaxies v3.9
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
66
energy distributions of starburst galaxies
Figure 2 — Spectral energy distributions of the obscured AGN in the models with increasing ionization
parameter from left to right (log U = −3.0, −2.0, −1.0). The total energy output has been scaled to
L = 1045 erg s−1 . Also shown are single temperature grey body fits to the 60–500 µm region.
(de Vaucouleurs et al. 1991). Many optical and near-IR fluxes were also taken from
Spinoglio et al. (1995) and the APM and 2MASS databases. The majority of 1–1300 µm
data points are from Klaas et al. (1997, 2001). Other data points were taken from
Sanders et al. (1988a), Sanders et al. (1988b), Murphy et al. (1996), Rigopoulou et al.
(1996), Duc et al. (1997), Benford (1999), Rigopoulou et al. (1999), Surace & Sanders
(2000), Lisenfeld et al. (2000), Dunne et al. (2000); Dunne & Eales (2001), Scoville et al.
(2000), Spoon et al. (2004) and references therein.
Where available the UV/optical and near-IR (JHK-band) points include aperture
corrections to allow a direct comparison with the larger aperture mid- and far-IR fluxes
(e.g., Spinoglio et al. 1995). All UV to near-IR fluxes have been corrected for Galactic
extinction using the E(B−V) values based upon IRAS 100 µm cirrus emission maps
(Schlegel, Finkbeiner, & Davis 1998) and extrapolating following Cardelli, Clayton, &
Mathis (1989).
2.2 High redshift sources
Because of their obscured nature only few “normal” SMGs have unambiguously identified counterparts and accurately determined redshifts. While the number of SMGs
with known redshifts is gradually increasing (Chapman et al. 2003a; Smail et al. 2004),
they are usually too faint to have an accurate sampling of their SEDs. Therefore, most
of the few high-redshift sources for which good data is available are quasars with a
significant contribution of beamed radiation from the active nucleus (e.g., Priddey &
McMahon 2001). Radio galaxies are similarly extreme sources, but for those the direct
glare from the AGN is thought to be blocked by an optically thick torus. They are better
suited for studying the galaxies themselves. Archibald et al. (2001) and Reuland et al.
(2004) observed 450 µm and 850 µm emission for 69 sources of this latter population
over a redshift range z=1–5. We selected, B3 J2330+3927, 4C 41.17, and 8C 1435+635
from amongst the combined sample, because they have the best coverage of the restframe far-IR and UV (observed R–K-band). The data were collected from Chambers
et al. (1990), Ivison et al. (1998), Archibald et al. (2001), and De Breuck et al. (2003, 2004).
Section 3. Infrared emission models and fitting procedures
Table 1 — Characteristic temperatures Tdust and β ’s for the AGN models
with ionization parameter U shown in Figure 2.
67
U
−3.0
−2.6
−2.3
−2.0
−1.6
−1.3
−1.0
−0.6
−0.3
0.0
Tdust
K
56.0
75.5
106.8
182.5
297.3
229.7
184.1
150.4
163.2
166.0
β
1.41
1.26
1.09
0.93
0.90
1.03
1.15
1.27
1.22
1.21
3 Infrared emission models and fitting procedures
In this paper we compare observational data to model SEDs consisting of both a starburst and an AGN related component. A detailed description of the starburst models
can be found in Dopita et al. (2004). Groves et al. (2004) present details of the AGN
related component. Below we highlight key ingredients of the models and present the
procedures used for the comparison with observations.
3.1 Starburst Models
3.1.1 General
The starbursts in the model are a composite of young H II regions and an underlying
older stellar population. The stellar populations are modeled with the stellar spectral
synthesis code STARBURST 99 (Leitherer et al. 1999). The models assume solar metalicity and the standard Salpeter initial mass function with a range in stellar masses
1 M < M < 120 M . For young (< 10 Myr) starbursts (the lifetime of the O and B
stars producing the ionizing radiation) the output of the stellar population code is fed
as input to the nebular modeling code MAPPINGS IIIq (Sutherland & Dopita 1993; Dopita et al. 2002a). The models include a 1-D dynamical evolution model of H II regions
around massive clusters and relevant dust and gas physics to provide the nebular line,
continuum and dust re-emission spectrum. Following Dopita et al. (2004) in total 104
star clusters of 104 M each are assumed to have formed over a period of 108 yr, normalized to an effective star formation rate of 1 M yr−1 . This normalized SED is then
scaled to the observed SEDs in order to estimate the star formation rates.
3.1.2 Dust and PAH implementation
Dust is modeled using solar abundances. The grain size distribution has a softened
power-law shape (with slope α = -3.3, close to the commonly used MRN model; Mathis
et al. 1977) with grain size limits of amin = 0.004 µm and amax = 0.25 µm for both
graphitic and silicaceous grains. The implemented distribution extends to smaller sizes
than the MRN model (0.01 µm for silicates). This results in a stronger dust re-emission
spectrum below 25 µm, to match the observed I RAS colors of starbursts (Dopita et al.
2004).
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
68
energy distributions of starburst galaxies
The models also include a simplified treatment of polycyclic aromatic hydrocarbons
(PAHs). PAHs are thought to be the main carriers of many observed IR absorption and
emission features. The precise composition of PAHs is still unknown. For simplicity
all PAHs are represented with a single type: coronene. The abundance of PAHs is implemented as a constant fraction (61%) of the total number of carbon atoms. However,
PAHs are fragile and can be destroyed by the harsh environments of galaxies hosting
starbursts or AGN (Voit 1992). Therefore, a dependence on a quantity analogous to
the ionization parameter is introduced. If the local radiation field per atom, defined
as H = FcnFUV
(the Habing Photodissociation Parameter; for details see Dopita et al. 2004)
H
> 0.005) then the PAHs are presumed destroyed and the
exceeds a critical value (H ∼
carbon they contain is returned to the gas.
Every embedded H II region has been modeled with MAPPINGSIIIq out to a column depth of NH = 1021.5 cm−2 . For the assumed dust template this corresponds to
AV ∼ 3 mag. This limit was chosen to recreate the PAH emission from photodissociation regions (PDRs) (see Dopita et al. 2004). The resulting spectrum is a superposition
of dust emission with a continuous range of temperatures arising at different distances
to the central source. The PAH features originate from beyond the distance where the
radiation field drops below the critical value.
3.1.3 ISM pressure and molecular cloud dissipation timescale
A fundamental ingredient of the models is that they include a dynamical description
of the individual H II regions. Young H II regions are envisaged to be small and deeply
embedded in their parental molecular clouds. The radiation and mechanical pressure
acting on the dust of the molecular cloud will lead to an expansion of the shell. This
expansion is slowed by the ambient pressure of the ISM following the formulation by
Oey & Clarke (1997). H II regions will remain smaller in higher pressure regions and
the dust grains will be closer to their central sources resulting in hotter dust. Figure
1 shows the cumulative SEDs of the older stellar population and the ensemble of H II
regions at their respective evolutionary states for ISM pressures P/k ∼ 104 , 106 , and
107 cm−3 K. The peak of the far-IR emission is shifted to shorter wavelengths for higher
pressures, indicating the higher average dust temperatures.
Additionally, with the expansion molecular clouds will become “leaky” to radiation
from the central source. In the models this is represented through the molecular cloud
dissipation time scale τclear . It is defined as the e-folding timescale over which the
covering factor f c of molecular clouds decrease:
f c (t) = exp(−t/τclear ).
For longer τclear the older H II regions will dominate the stellar continuum in the SEDs.
Furthermore, longer τclear increase the far-IR fluxes relative to a flatter near-IR stellar
continuum, broaden the far-IR bump, and strengthen the PAH features.
The range of temperatures in the models is physically motivated. Increasing τ clear
partially counteracts increasing the pressure, because the older H II regions are larger
and will increase the emission from cool dust. The resulting degeneracy is not very
strong, because the temperature is much more sensitive to the pressure than to the
molecular cloud dissipation time scale.
Section 3. Infrared emission models and fitting procedures
69
3.1.4 Visual extinction
Radiation from the stars that escapes the local H II regions through the “holes” in the
dusty shells, may encounter an outer foreground dusty screen. To take this into account, additional attenuation is applied according to a theoretical law similar to the
one presented by Fischera et al. (2003) but for the current dust model. This model reproduces the observed Calzetti attenuation law for starbursts closely (Calzetti 2001). It
assumes a turbulent screen with a log-normal distribution and that PAHs are the main
carriers of the 2175 Å absorption feature. The PAHs are destroyed within the strong
UV radiation fields of the diffuse neutral or ionized ISM, explaining the absence of the
2175 Å feature in starbursts. In the models the energy absorbed by this foreground
screen is not re-emitted in the FIR but “lost”. This approximation is justified for obscured sources such as ULIRGs because only little UV/optical radiation escapes from
the H II region and the largest fraction has already been reprocessed. Therefore, the
effective visual attenuation is essentially an independent parameter. It mostly affects
the UV through near-IR part of the SED.
3.2 AGN torus models
The AGN contribution is modeled as a deeply embedded dusty narrow line region.
We used a power-law ionizing source of luminosity LAGN and spectral index α = −1.4
as input to the MAPPINGS code with density n = 104 cm−3 , out to a column depth
of 1021.5 cm−2 . Model SEDs for different ionization parameters in the range −3.0 ≤
log U ≤ −1.0 are shown in Figure 2. To first order, the FIR portions of these SEDs can
be represented by single temperature modified black bodies with dust emissivity proportional to λ−β . The effective dust temperatures and β ’s are given in Table 1. Clearly,
this is not a full description of an embedded AGN, as it describes only reprocessed
radiation and no direct radiation.
Some ULIRGs show a clear radiation excess in the near-IR to mid-IR regime. This
cannot be accounted for with the obscured AGN model described above. It is likely
that for these galaxies the torus surrounding the AGN (as required by orientation based
AGN unification models; e.g., Barthel 1989) is inclined such that direct radiation from
the AGN and radiation from hot dust close to core become visible.
Many authors have described emission from inclined dusty tori (e.g., Pier & Krolik 1992; Granato & Danese 1994; Efstathiou & Rowan-Robinson 1995; van Bemmel &
Dullemond 2003). However, the relevant physics for dusty tori is complex and such
models add a significant number of assumptions and parameters. These cannot be
properly constrained for most of the galaxies in the present sample. The goal of this
paper is to obtain a physical understanding of the parameters controlling the starburst
related SEDs of ULIRGs. Therefore, we have not aimed to fit the 3–15 µm portion of
the SEDs, but rather use theoretical SEDs based on relatively simple physical considerations. We obtain an estimate of the AGN related fraction to the far-IR emission from
the fitted dusty narrow-line region. This simple approach is acceptable for estimating
the ratios of starburst and AGN related far-IR emission because the largest fraction of
AGN related far-IR emission in obscured sources is likely reprocessed emission from
the cool outer regions of the narrow-line region. Obviously, it is not suitable for esti-
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
70
energy distributions of starburst galaxies
Name
IRAS 05189
IRAS 12112
Mrk 231
Mrk 273
IRAS 14348
IRAS 15250
Arp 220
IRAS 22491
LTAH
bol
L
LF03
bol
L
Lbol
L
SFRTAH
M yr−1
SFRF03
M yr−1
SFRSB
M yr−1
SFRAGN
M yr−1
ATAH
V
mag
AV
mag
F03
Fagn
Fagn
3.0 × 1012
1.7 × 1012
3.4 × 1012
1.4 × 1012
2.2 × 1012
1.0 × 1012
1.5 × 1012
1.2 × 1012
1.1 × 1012
1.6 × 1012
3.0 × 1012
1.1 × 1012
1.6 × 1012
1.0 × 1012
1.1 × 1012
1.1 × 1012
1.4 × 1012
2.3 × 1012
3.3 × 1012
1.6 × 1012
2.4 × 1012
1.3 × 1012
1.6 × 1012
1.7 × 1012
174
268
557
228
360
203
286
242
136
313
376
208
248
56
192
48
407
655
959
336
655
277
370
447
229
229
541
208
142
117
277
142
4.7
5.4
2.8
4.9
5.4
4.5
5.4
4.4
9.0
6.6
6.8
9.0
9.0
9.0
5.2
9.0
0.3
0.0
0.3
0.1
0.4
0.6
0.0
0.7
0.1
0.4
0.1
0.3
0.6
0.4
0.0
0.5
Table 2 — Summary of SED fitting results from this paper and work by Takagi et al. (2003,indicated by
index TAH) and Farrah et al. (2003,indicated by F03). Total inferred bolometric luminosities L bol , star
formation rates, visual attenuation, and fractional contribution of AGN to the total IR luminosities are
shown (converted to our adopted cosmology).
Figure 3 — Distributions of the dust temperatures (left panel) and dust emissivity proportionality parameter β (right panel). These were inferred from single-temperature modified blackbody fits to the
60–500 µm regime of the best-fit SEDs using SB fits (light grey) and SB+AGN fits (dark grey).
mating the bolometric AGN luminosity because any direct radiation is not considered.
When strong emission in the 3–15 µm range is present, our models cannot be applied.
3.3 The fitting procedures
Comparing the models to the observations consisted of two steps. First, the observed
data were shifted to rest-frame frequencies. Secondly, we constructed a grid of model
SEDs for the full set of parameter ranges:
•
•
•
•
•
log P/k = 4, 6, 7 cm−3 K,
τclear = 1, 2, 4, 8, 16, 32, 100 Myr,
AV = 1.0–9.0 (41 discrete values),
SFR = 50–5000 M yr−1 (steps of 10% increase),
log U = −3.0–0.0 (10 discrete values),
Section 4. Results
71
• LAGN = 1043 –1047 erg s−1 (steps of 50% increase).
This grid was then searched to obtain the least squares solution of the differences between the observations and all realizations of the model. This fitting method gives
most weight to the most energetic data points. For obscured starbursts which emit
most radiation in the FIR it is therefore analogous to requiring that the total energy
in the starburst is reproduced by the model. As we will see below this means that
UV–near-IR data points will be fitted less accurately.
We have performed three sets of fits for the full range of parameter space each. Each
set was aimed at obtaining a good fit at the following wavelength regimes:
• FIR-fits: starburst-only fits to the 60–1300 µm range,
• SB-fits: similar to FIR-fits but also include UV–near-IR (0.366–2.2 µm) and 25–
60 µm data,
• AGN+SB-fits: similar to SB-fits but also include mid-IR (15–25 µm) data and an
obscured AGN component.
The FIR-fits attempt to match the peak of the far-IR emission and the slope of the
far-IR tail only. This is comparable to the common practice in submillimeter astronomy of inferring star formation rates from modified blackbody fits to the far-IR SEDs
of SMGs. A very weak handle on the molecular cloud dissipation time scale can be
obtained from the width of the IR bump. The SB-fits include the near-IR data to get
a good estimate of the molecular cloud dissipation time-scale and therefore a much
improved estimate of the star formation rate. The AGN+SB fits includes also the dusty
narrow-line region in order to obtain an estimate of the “contamination” of the far-IR
regime by AGN related emission.
Because our models are descriptive, but have not quite the level of sophistication
to match observations in detail, we do not quote formal uncertainties on the estimated
parameters. From comparing neighboring fits it is found that the uncertainties in the
star formation rates are of order ∼ 50%. The molecular cloud dissipation timescales
are accurate to a within a factor of two. The pressure estimates appear not subject to
much uncertainty. However, this is a consequence of the sparse sampling of the grid
in pressure-space.
4 Results
4.1 General Results
Table 3 presents an overview of the best-fit parameters for the three sets of fits. The
results of the AGN+SB-fits are visualized in Figure 8 with the fit parameters indicated.
Despite the relative simplicity of the models, it is found that generally they can fit
the observations well. The main findings are itemized below:
• High pressure models P/k ≥ 106 cm−3 K are required in all cases,
• The molecular cloud dissipation timescales are long > 8 Myr,
• Most star formation rates are in the range 100–250 M yr−1 but a few are much
higher,
• Inferred values for AV span the entire range probed,
• There is a large spread in the fractional contribution of AGN to the 60–1300 µm
regime.
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
72
energy distributions of starburst galaxies
Figure 4 — Distributions of the inferred molecular cloud dissipation time
scales for SB+AGN fits. Circles indicate the full sample, triangles indicate
luminous galaxies with LFIR ≥ 6.3 ×
1011 L , and squares indicate galaxies
with LFIR < 6.3 × 1011 L .
Not all wavelength regimes could be fitted equally well. In approximately 30%
of the cases it proved impossible to fit the UV datapoints. These UV components
may arise from outer stellar populations that are less obscured than the central starburst. The PAH features cannot be reproduced accurately for the Seyfert sources with
an almost flat 3 − 15 µm SED (05189−2524, Mrk231, Mrk273, Mrk463, 15462−0450,
19254−7254). Adding a strong AGN-related component would destroy some PAHs
and through an increase of the mid-IR emission hide any features that are left, as
required by the observations. Interestingly, there is also a large discrepancy for the
archetypical ULIRG Arp 220 for which no signature of an AGN component has been
found. The models predict an order of magnitude more PAHs than are observed. In
this case the very high star formation rate density may be responsible. It could provide a strong photodissociating UV field destroying a large fraction of the PAHs. For
all other sources the SEDs are well matched and produce approximately the correct
amount of PAH emission.
4.2 Comparison with literature
In order to quantitatively investigate how well the models reproduce the observations,
a single temperature modified blackbody spectrum with emissivity proportional to
λ−β was fitted over the region 60–500 µm. Figure 3 shows the effective temperature
and emissivity proportionality coefficients for both SB and SB+AGN fits. All SB fits
yield a best-fit value β = 1.5, with effective temperatures T = 37–48 K (see Table 3).
This is in good agreement with what has been found for single temperature fits to
bright IRAS galaxies (Dunne et al. 2000). In the SB+AGN fits we have included a
hotter component with effective temperatures T ∼ 55–160 K (see Table 1 and Fig. 2).
This improves the fits although there is obvious mid-IR excess that is not accounted for
in this simple parameterization. In this case we infer temperatures T = 37–57 K for the
Section 5. Discussion
73
Figure 5 — Left: The fractional contribution of the AGN related component to L FIR as a function of
the ratio of mid-IR and far-IR emission. As expected the AGN fraction rises with increasing mid-IR
emission. Middle: The ionization parameter of the AGN as a function of mid-IR to far-IR emission.
Right: The relative fraction of far-IR emission that is related to the embedded AGN as a function of L FIR .
single temperature fits with slightly lower values β = 0.9–1.4.
Below we compare our results with the results of similar analyses by Takagi et al.
(2003) and Farrah et al. (2003) which provide independent estimates for some of the parameters explored in this paper. Takagi et al. (2003) have developed fairly sophisticated
SED models for star forming galaxies. In their models the temperature is controlled by
the compactness parameter of the galaxies. Deeper potential wells lead to more compact galaxies and higher temperatures. Deeper potential wells would also increase the
pressure. Therefore compactness is a tracer of the pressure considered in this paper.
The models by Takagi et al. provided an estimate of the visual extinction: the column
depth to the starburst region is related to the compactness. Farrah et al. (2003) explored
the fractional AGN contribution of ULIRGs, using evolutionary models by Efstathiou
et al. (2000).
In Table 2 we present the results of the fits by Takagi et al. and Farrah et al. vis-à-vis
our results for those sources that were included in all three programs. The inferred
bolometric luminosities Lbol , and SFRs for the AGN+SB models match the results from
Takagi et al. and Farrah et al. to within a factor of two. Especially for Arp 220 and
Mrk 231 the three different models agree closely. For Arp 220 the inferred A V and
Fagn are also closely matched, but for the other sources the matches are approximate
only. Given the very different methods to construct the theoretical SEDs, this result is
promising.
5 Discussion
5.1 Limitations of the models
While our models generally do reproduce the SEDs well, our approach does have some
limitations.
• The 3–15 µm regime is not fitted, resulting in bad fits for sources in which the
AGN is not fully obscured. If there is strong emission in this regime, the models
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
74
energy distributions of starburst galaxies
•
•
•
•
are not applicable.
No ultra-compact H II regions have been included. We might reasonably expect
to find relatively more of these in high-pressure regions. This could be responsible for warm dust 12–30 micron components such as seen in Arp 220, and are
now often attributed to AGN related emission.
No cool cirrus component has been included.
The models assume that all starbursts have an age of 100 Myr. No luminosity or
metalicity evolution in the starburst has been incorporated.
Galaxies are essentially treated as point-sources and less obscured “external” stellar populations are not considered. For ULIRGs the star forming regions are
usually compact. However, the data were collected in large apertures possibly
explaining why it was not always possible to fit the UV data points.
5.2 Pressure
All of the fits for the ULIRGs have far-IR bumps which peak below 100 µm and therefore are characterized by high pressures, P/k > 106 cm−3 K (see Fig. 1 and 8). This
result is not unexpected, because the star formation region can only be as compact as
it is in ULIRGs if the star formation is occurring in a very high pressure and high density environment. An independent estimate of the pressure in the H II regions can be
obtained through measurements of the density-sensitive [S II] λ6717, 6731 Å doublet.
Kewley et al. have applied this technique to warm I RAS galaxies and indeed, assuming T ∼ 104 K, pressures P/k > 106 cm−3 K are inferred for most objects (Kewley et al.
2001a,b).
In the local universe, the dust temperature (but not the amount of dust) inferred
from the modified blackbody fits to the far-IR bump in starburst galaxies is found to
correlate with the absolute luminosity (Dunne & Eales 2001). High-redshift SMGs display a similar correlation, but shifted to higher luminosity (Blain et al. 2004). At a given
luminosity, the dust in SMGs is about 20 K cooler than in ULIRGs in the local universe,
and at a given dust temperature, the SMGs are typically 30 times more luminous than
their ULIRG counterparts.
What could this mean? Takagi et al. (2003) have found that most ULIRGs have a
constant surface brightness of order 1012 L kpc−2 (the few objects with higher surface
brightnesses may be post-merger systems). Our results show that this corresponds to
an ISM with a pressure of order P/k ∼ 107 cm−3 K. These parameters probably characterize a maximal star formation density, above which gas is blown out into the halo
of the galaxy and star formation suppressed. Possibly only mergers, which provide
an additional ram-pressure confinement of the star formation activity, can exceed this
surface brightness. Thus, in order to scale the star formation up to the rates inferred for
SMGs (∼ 1000–5000 M yr−1 ), a greater area of the galaxy must be involved in star formation, rather than more star formation occurring in the same volume. To reach a typical far-IR luminosity of 1013 L for this upper limit to the star formation rate density,
we require star formation over an area of ∼ 10 kpc2 . For the most luminous SMGs star
formation must be extended over an area of at least ∼ 100 kpc2 . Indeed, this is exactly
what is found from high resolution optical and radio imaging of 12 luminous SMGs at
z ∼ 2. Chapman et al. (2004b) report star formation rates of ∼ 1700 M yr−1 occurring
Section 5. Discussion
75
within regions of ∼40 kpc2 , implying star formation rate densities of 45 M yr−1 kpc−2 .
This is comparable to the upper limit found for local starburst galaxies (Meurer et al.
1997) and argues that:
• SMGs host starbursts extended on galaxy-wide scales, rather than the more compact or nuclear starbursts typical of ULIRGs in the local universe.
• Small-scale physical mechanisms that limit the star formation process in SMGs
are similar to those operating in the most vigorous systems locally.
• Because of the greater physical extent inferred for the starburst region in the
SMGs, the modeling parameters we have derived for local ULIRGs: pressure,
molecular gas dissipation timescales, and line of sight attenuations can probably
be directly applied to modeling SMGs in the distant universe.
5.3 Molecular cloud dissipation time scale
Figure 4 shows a histogram of the inferred molecular cloud dissipation timescales τ clear .
This is the timescale on which molecular clouds are destroyed, or, equivalently, the
timescale taken for stars to escape from the dense regions of star formation and molecular clouds. It effectively determines the fraction of UV/visual radiation which is intercepted by dust in dense clouds and re-radiated in the far-IR. Therefore it controls
the offset between the near-IR and far-IR peaks. AGN+SB fits generally imply longer
τclear than SB fits. This is because an AGN component reduces the fraction of near-IR
attributed to the starburst, while the level of the far-IR peak remains mostly related
to the starburst. For AGN dominated sources τclear cannot be reliably determined, because the near-IR is no longer representative of the starburst.
>
The histogram shows that there is a preference for long dissipation timescales τ clear ∼
32 Myr. For such long timescales the dust screen absorbs almost all UV/optical radiation and effectively acts as a bolometer. It is also found that the galaxies with L FIR
> 6 × 1011 L have longer τclear than lower luminosity sources. This agrees with the
∼
result found by Takagi et al. (2003) for ULIRGs, that more active starburst regions tend
to be more heavily obscured. More fundamentally, it has been observed also for more
regular galaxies that dust obscuration increases both with galaxy mass (Kewley et al.
2002), and with the absolute rate of star formation (e.g., Adelberger & Steidel 2000;
Dopita et al. 2002b). The result that τclear increases with increasing LFIR shows that
the common practice of assuming that for SMGs and other extreme starbursts all light
from the starburst is reprocessed to far-IR wavelengths is not unreasonable.
5.4 AGN contribution to MIR and FIR wavelengths and its influence on the inferred star formation rates
There is a longstanding controversy whether AGN or starbursts contribute most to
the IR emission of ULIRGs and HyLIRGs. For example, Veilleux et al. (1995) found
from optical spectra that the fraction of I RAS sources with AGN-like emission line
ratios increases with increasing IR luminosity, up to ∼60% at LIR > 1012 L . In contrast,
results from mid-IR spectroscopy show that most ULIRGs (∼80%) are powered mainly
by starbursts (Genzel et al. 1998; Rigopoulou et al. 1999). More recent observations
from near-IR spectroscopy and ISO still give contrasting results (e.g., Veilleux et al.
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
76
energy distributions of starburst galaxies
Figure 6 — The inferred star formation
rates as a function of total far-IR (60–
1300 micron) luminosity. Circles indicate the SFRs derived using fits to the
far-IR part of the SED only, triangles indicate SFRs inferred from fits to both
the near-IR and far-IR, and squares indicate inferred SFRs after including a
AGN fraction. The open stars and
“best-fit” line indicate the results of SB
fits with a fixed molecular cloud dissipation timescale τclear = 8 Myr. Fits to
both the near-IR and far-IR yield lower
inferred SFRs than fits to the FIR region
only.
1999; Tran et al. 2001).
From our modeling we find that both starburst and AGN activity are crucial to our
understanding of the ULIRG SEDs. Figure 5 shows three results from our program. It
is found that:
• The fractional contribution of AGN emission to LFIR is correlated with LMIR / LFIR .
This is because we used LMIR to scale the narrow-line region.
• As expected, the ionization parameter log U (equivalent to dust temperature) increases for larger ratios of LMIR / LFIR .
• The fractional contribution of AGN activity is not correlated with LFIR , but almost
always less than ∼ 60%.
Commonly, it is assumed that the AGN contribution to LFIR is negligible (e.g., Farrah et al. 2002). However from our analysis this is found not to be strictly true. AGN
related emission can constitute more than 40% of LFIR in approximately half of the
ULIRGs. This is similar to what was found recently by Prouton et al. (2004).
Independent estimates of the AGN contribution could be obtained both through obtaining independent measures of the star formation rate and AGN activity. This could
be done through comparison with X-ray observations (e.g., Lutz et al. 2004), reddening
corrected optical emission line ratios (e.g., Kewley et al. 2002), relative strengths of the
so-called unidentified infrared bands (UIBs) and mid-IR continuum (e.g., Laurent et al.
2000), and sensitive radio observations Prouton et al. (2004). Once such data for a large
number of the galaxies of the present sample becomes available, they could provide an
important check on our models.
The main driver of this paper is to investigate how inferred star formation rates
depend on the SEDs of starburst galaxies. In Figure 6 we have plotted the inferred
star formation rates as function of LFIR for FIR-fits, SB-fits, and SB+AGN-fits. It shows
that increasing the molecular cloud dissipation time scales τ clear lowers the inferred
Section 5. Discussion
77
Figure 7 — Spectral energy distributions of HzRGs, ordered by RA. Upper limits are indicated by a
downward arrow. The dotted lines represent modified blackbody fits.
star formation rates significantly. In the FIR-fits these timescales are constrained only
by the long-wavelength slope of the FIR-peak. This results in short τclear because there
is a trend for the slopes to be rather steep. Lower pressures and longer τ clear are indistinguishable without a good sampling of the far-IR bump. This illustrates the importance of sensitive mid-IR data for constraining the inferred star formation rates. Taking
the near-IR points into account with the SB-fits results in τclear with a median value of
∼ 8 Myr (indicated by the line). Above we argued that AGN frequently may contribute
up to 40% to the total LFIR . Indeed, Figure 6 shows that taking an AGN component into
account often lowers the derived star formation rates by a factor of ∼2–3.
5.5 Star formation at high redshift
Recently, Blain et al. (2004) investigated a sample ranging from local ULIRGs to distant
(up to z ∼ 5) rest-frame far-IR luminous galaxies. They reported a correlation between
the inferred dust temperature and the luminosity of the host galaxy. In the context of
this paper, this could indicate a correlation between star formation rate and pressure
in the ISM of the galaxies.
To investigate how the characteristics of nearby starbursting galaxies translate to
high redshift, we have modeled the SEDs of three z > 3 radio galaxies. The results of
SB fits to their SEDs are shown in Figure 7. Star formation rates > 4000 M yr−1 are
inferred. If continued over a dynamical timescale (∼ 108 yr) such bursts would convert
an amount of gas equivalent to the mass of a L∗ galaxy into stars.
Due to the sparse wavelength sampling of the radio galaxy SEDs the fits cannot be
tightly constrained. Especially, it is not possible to quantify any AGN contribution to
the FIR, despite the fact that a highly active AGN is obviously present. Yet, the farIR slope for 4C 41.17 (and possibly also for 8C 1435+635) is steeper than can be fitted
with P/k ∼ 107 cm−3 K. It seems that to fit the SEDs of HzRGs even higher pressures
are required. Such high pressures could be the consequence of the radio source, providing an additional pressure component and possibly inducing star formation (e.g.,
Begelman & Cioffi 1989; Bicknell et al. 2000).
Assuming equipartition (analogous to a minimum energy requirement), it is possible to calculate lower limits to the pressures associated with the radio emission. This
requires estimates of the volume, radio power, and spectral indices of the radio lobes.
Using the radio observations by Carilli et al. (1994) and Lacy et al. (1994) for 4C 41.17
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
78
energy distributions of starburst galaxies
> 0.3 − 1.2 × 107 cm−3 K are inferred at disand 8C1435+635, typical pressures of P/k ∼
tances of > 50 kpc from the center. These high pressures would lead to even more
compact H II regions and correspondingly higher typical dust temperatures. Another
explanation for the high pressures could be related to mergers. Hubble space telescope
observations shows that HzRGs consists of many clumps that will merge with the central growing galaxy over dynamical timescales of ∼ 108 yr (e.g., Pentericci et al. 1998).
Optical evidence for elongated structures in SMGs are also interpreted as evidence for
ongoing merger activity (Chapman et al. 2003b). But for SMGs the mergers seem more
moderate and comparable to those seen in ULIRGs.
These high pressures would increase the typical temperatures and could help shape
the IR-luminosity-temperature relation. Such hot dust would radiate more efficiently.
Together with a possible AGN contribution this indicates that star formation rates often quoted for high redshift objects (for which usually only rest-frame far-IR data are
available) might have to be lowered significantly.
6 Conclusion
We have compared theoretical SED models for starburst regions with observations of
ULIRGs and HzRGs. These models include a sophisticated treatment of dust and a
physically motivated evolution of the individual H II regions. With these models we
investigated the properties of these galaxies, such as the SFR, optical depth, molecular
cloud dissipation time scale and pressure of the ISM. For the starburst galaxies, the inferred SFRs and optical depths match values from the literature closely. This indicates
that the models can be used to derive these properties from the SEDs.
We have included a dusty narrow-line region to help quantify the maximum contribution of hidden nuclear activity to the inferred SFR. From this analysis we find that
more luminous sources have a higher absolute contribution from AGN to L FIR , but
that the fractional AGN contribution does not increase systematically with increasing
luminosity. For ∼ 50% (20/41) of the galaxies we find a clear excess of MIR emission,
requiring a significant (> 35% to LFIR ) contribution of a hidden AGN. In 6 galaxies
there is evidence for a power-law spectrum over the UV/optical to FIR range. This is
likely related to an AGN which is less obscured than we have modeled. The inferred
star formation rates including hidden AGN activity are lower by a factor of 2–3 as
compared with models attributing all of the far-IR emission to starburst activity.
From our investigation, it is found that almost all ULIRGs can be best fitted with
high-pressure (P/k ∼ 106−7 cm−3 K) models. Such pressures are much higher than seen
normally in the ISM and most likely indicate the importance of dynamical interactions
associated with galaxy merging in triggering ULIRGs. We also find that the sources
with higher star formation rates are best fitted with models having longer molecular
cloud dissipation time scales. This confirms the result found by Takagi et al. (2003) that
more active starburst regions tend to be more heavily obscured.
Because ISM densities/pressures may have been higher in the early universe, one
might expect high redshift sources to have higher effective temperatures. Evidence
for such hot, dusty ultraluminous galaxies at redshift of z ∼ 2 has been presented by
Chapman et al. (2004a). A significant fraction of these may host a dominant AGN
Section 6. Conclusion
79
(∼ 20%) or be starburst/AGN hybrids (∼ 25%), but the remainder shows no detectable
signs of AGN. The high temperatures in the latter population could be the result of the
processes described in this paper.
Higher temperatures would lower the amount of dust for a given far-IR luminosity.
Together with a possibly significant (up to > 60%) fraction of AGN related far-IR emission, this indicates that the star formation rates often quoted for high redshift objects
(for which until recently only rest-frame far-IR data have been available) may have to
be lowered by a factor ∼ 3. Accurate sampling of the MIR to FIR region of the SEDs
such as Spitzer and ALMA will provide is essential to obtain reliable star formation
rates.
Acknowledgements
We thank Christian Kaiser, Bram Venemans, Andrew Zirm, and Michelle Cappellari
for helpful discussions. This work was performed under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of
California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng48. M.D. acknowledges the support of the ANU and the Australian Research Council
(ARC) for his ARC Australian Federation Fellowship, and also under the ARC Discovery project DP0208445. M.R. acknowledges the support of a travel grant through
M.D.’s ARC Australian Federation Fellowship Support Grant and thanks everybody at
the Mt. Stromlo observatories for a very productive visit. This research has made use
of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National
Aeronautics and Space Administration.
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81
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
82
energy distributions of starburst galaxies
Figure 8 — Best fit spectral energy distributions of the ULIRGs in our sample, ordered by RA. In each
case the solid line is the combined SB+AGN best fit model. Observed data points are overlaid with
upper limits indicated by a downward arrow. P/k represents the pressure in cm−3 K, τ the molecular
cloud dissipation timescale in Myr, SFR the star formation rate in M yr−1 , AV the visual attenuation,
and Fagn the fractional contribution of AGN related emission to the total 60–1300 µm luminosity. The
dashed lines represent modified blackbody fits over the range 60–500 µm.
Section 6. Conclusion
83
Figure 8 — continued.
Chapter 5. The influence of ISM characteristics and AGN activity on the far infrared spectral
84
energy distributions of starburst galaxies
Figure 8 — continued.
0.0963
0.0971
0.3422
0.0778
0.1343
0.3100
0.0799
0.0425
0.0794
0.0924
0.0723
0.0417
0.0373
0.0506
0.0811
0.0676
0.0756
0.0553
0.0182
0.1005
0.1334
0.0245
0.0424
0.3411
0.0660
0.0788
0.0615
0.0995
0.0845
0.1055
0.1295
0.0870
0.0426
0.0925
0.0775
0.0760
0.0446
0.1062
0.0645
0.0927
0.3347
z
D
Mpc
398
401
1338
323
551
1221
331
177
329
382
300
174
156
211
336
281
314
230
76
415
547
102
177
1334
274
327
256
411
350
435
532
360
178
383
322
315
186
438
268
383
1311
SFRFIR
M yr−1
407
492
3012
407
872
2262
336
595
655
655
959
1405
595
142
959
872
370
595
370
407
872
117
793
2738
655
655
407
447
370
793
1055
959
595
277
541
872
447
872
595
595
1870
SFRSB
M yr−1
407
370
2057
229
872
2262
336
407
447
492
655
959
336
172
655
447
305
277
370
407
872
189
595
1870
655
336
407
447
305
793
1055
959
277
277
252
447
305
595
407
407
1277
SFRSB+AGN
M yr−1
407
88
50
156
336
793
189
229
208
142
229
541
208
129
142
88
252
117
277
107
229
208
305
370
370
97
252
305
189
172
595
80
80
107
107
142
117
117
156
156
189
P/kFIR
cm−3 K
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
P/kSB
cm−3 K
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
P/kSB+AGN
cm−3 K
7
6
4
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
7
6
7
7
6
7
7
6
7
4
7
6
7
6
7
7
7
7
7
7
7
τ FIR
Myr
100
4
2
4
8
4
8
2
4
4
4
2
4
2
4
2
8
2
16
8
16
100
8
2
8
2
4
100
8
8
8
2
2
100
2
2
4
4
4
4
4
τ SB
Myr
100
8
4
16
8
4
8
4
8
8
8
4
16
2
8
8
16
8
16
8
16
8
16
4
8
8
4
100
16
8
8
2
8
100
8
8
8
8
8
8
8
τ SB+AGN
Myr
100
32
100
100
100
100
32
8
32
100
100
8
16
2
100
100
16
16
100
32
100
4
100
8
32
100
8
32
100
32
32
2
32
32
32
16
100
100
100
100
100
-0.6
-2.6
-3.0
-0.6
-2.6
-2.3
-2.3
-1.6
-2.6
-3.0
-3.0
-1.6
-2.6
-0.6
-3.0
-3.0
-3.0
-2.3
-2.3
-2.6
-3.0
-1.6
-3.0
-3.0
-2.6
-2.6
-0.6
-3.0
-2.3
-3.0
-2.3
-3.0
-2.6
-3.0
-2.6
-2.6
-2.6
-3.0
-2.6
-3.0
-3.0
U
L AGN
erg s−1
45.47
46.87
47.58
45.29
47.05
47.23
46.35
46.52
46.70
46.87
46.87
46.87
46.70
46.17
47.05
46.87
46.17
46.70
45.99
46.87
47.23
45.82
46.87
47.40
46.70
46.70
46.35
46.87
46.52
47.23
46.87
47.05
46.70
46.87
46.52
46.87
46.52
47.05
46.70
46.70
47.40
TdSB
K
37.31
39.68
42.52
38.34
39.68
43.02
39.78
42.52
39.68
39.68
39.68
42.52
38.34
48.02
39.68
39.68
38.36
39.68
38.34
39.68
38.34
40.04
38.34
42.52
34.00
39.68
42.52
37.31
38.51
39.68
39.68
48.14
39.68
37.55
39.68
39.68
39.68
39.68
39.68
39.68
39.68
1.50
1.49
1.49
1.50
1.49
1.48
1.49
1.49
1.49
1.49
1.49
1.49
1.50
1.49
1.49
1.49
1.50
1.49
1.50
1.49
1.50
1.49
1.50
1.49
0.93
1.49
1.49
1.50
1.49
1.49
1.49
1.49
1.49
1.50
1.49
1.49
1.49
1.49
1.49
1.49
1.49
β SB
TdSB+AGN
K
37.69
52.27
54.22
37.59
44.55
42.38
41.04
43.78
43.66
45.13
42.89
43.75
44.64
53.80
46.46
47.04
39.92
48.75
38.34
49.65
46.54
38.14
41.76
49.14
36.51
47.03
41.33
39.58
41.50
56.32
40.87
53.77
49.90
47.48
45.09
49.61
43.73
47.76
44.28
42.96
49.22
1.49
0.53
1.40
1.50
1.36
1.38
1.41
1.39
1.38
1.40
1.41
1.39
1.37
1.40
1.39
1.39
1.47
1.29
1.47
0.54
1.39
0.96
1.43
1.41
0.74
1.32
1.45
0.71
1.40
0.29
1.42
1.34
1.30
0.67
1.36
1.32
1.37
1.38
1.36
1.41
1.38
β SB+AGN
Table 3 — Summary of the best-fit parameters for fits to the SEDs of the 41 ULIRGs shown in Figure 8 ordered by RA. The colums show the
source name, redshift, distance, star formation rate, the logarithm of the pressure of the interstellar medium, molecular cloud dissipation timescale,
ionization parameter, total luminosity of the reprocessed AGN light, and temperature obtained with modified blackbody fit to the regime 60–500 µm
with proportionality index β . The indices FIR, SB, and SB+AGN refer to the fitting procedures outlined in §3.3.
00199-7426
00262+4251
00406-3127
03068-5346
03158+4227
03538-6432
04232+1436
05189-2524
06035-7102
06206-6315
12112+0305
Mrk231
Mrk273
Mrk463
14348-1447
14378-3651
15245+1019
15250+3609
Arp220
15462-0450
16090-0139
NGC6240
17208-0014
17463+5806
18090+0130
18470+3234
19254-7245
19458+0944
20046-0623
20087-0308
20100-4156
20414-1651
ESO286-19
21130-4446
21504-0628
22491-1808
ESO148-2
23230-6926
23365+3604
23389-6139
23515-3127
Name
Section 6. Conclusion
85
Chapter 6
Giant Lyα nebulae associated with high
redshift radio galaxies
Michiel Reuland, Wil van Breugel, Huub Röttgering, Wim de Vries, S.A. Stanford, Arjun
Dey, Mark Lacy, Joss Bland-Hawthorn, Michael Dopita, and George Miley, The
Astrophysical Journal, Vol. 592, p. 755, 2003
We report deep Keck narrow-band Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100−200 kpc scale
emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the
outer regions, radially directed filaments, cone-shaped structures and (indirect)
evidence for extended Lyα absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter
(CDM) halos, forming stars and multiple stellar systems. Merging of these “building” blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful
radio jets and lobes. The radio sources, starburst superwinds and AGN radiation
then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lyα nebulae.
1 Introduction
H
z > 2) are great beacons for pinpointing some
of the most massive objects in the early universe, whether these are galaxies,
super-massive black holes or even clusters of galaxies (van Breugel et al. 2002).
Powerful, non-thermal radio sources are uniqely associated with massive, multi L ?
elliptical galaxies. At low redshifts this has been known since the first optical identifications of extra-galactic radio sources were made possible using radio interferometers (Cygnus A; Carilli & Barthel 1996). The large, twin-jet, double-lobe morphologies
and enormous radio luminosities (P178 MHz ∼ 5 × 1035 erg s−1 Hz−1 ) suggested already
early on that such galaxies must have spinning, super-massive black holes (SMBHs) in
their centers (Rees 1978; Blandford & Payne 1982). We now know that the masses
of the stellar bulges of galaxies and their central black holes are closely correlated
(MSMBH ∼ 0.006Mbulge ; Magorrian et al. 1998; Gebhardt et al. 2000; Ferrarese & Merritt 2000), indicating a causal connection and it is no longer a surprise that their parent
> 2 × 1011 M ).
galaxies occupy the upper end of the galaxy mass function (∼
IGH redshift radio galaxies (HzRGs;
87
88
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
At high redshifts the evidence is more recent but equally compelling. The combined
near-infrared “Hubble” K − z relation for radio and field galaxies (De Breuck et al. 2002;
Jarvis et al. 2001) shows that radio-loud galaxies are the most luminous at any redshift
0 < z < 5.2. This is despite considerable changes in their rest-frame morphologies from
smooth ellipticals at z ∼ 1 (Best, Longair, & Röttgering 1998) to large (∼ 50 − 70 kpc)
multi-component, structures aligned with the radio source at z > 2 (Pentericci et al.
1999; van Breugel et al. 1998).
Evidence that the HzRGs are young forming galaxies has been provided by the
direct detection of absorption lines and P Cygni-like features from young hot stars
(Dey et al. 1997) and strong sub-mm continuum emission (Dunlop et al. 1994; Ivison
et al. 1998; Archibald et al. 2001; Reuland et al. 2004). The sub-mm observations im> 1013 L ) and huge
ply “hyper” luminous rest-frame far-infrared luminosities (L FIR ∼
(> 1000 M yr−1 ) star formation rates (SFRs). Large amounts of star formation are
also indicated by the recent detections in several HzRGs of very extended (∼ 30 − 50
kpc) molecular gas and dust clouds (Papadopoulos et al. 2000; De Breuck et al. 2002b),
showing that the star formation in these systems occurs on galaxy wide scales.
There are great practical advantages in using HzRGs to study the formation and coevolution of massive galaxies and their central black holes. Radio source identifications
are unbiased with respect to heavy obscuration by dust, which is especially important
in young galaxies with large amounts of star formation. Furthermore, HzRGs are the
most extended and luminous galaxy-sized structures at many wavelengths and can
therefore be studied in great detail over nearly the entire electromagnetic spectrum.
Some specific questions that one might hope to address are: Do the stellar bulges of
massive elliptical galaxies form mostly during one major, “monolithic” collapse or over
a longer period through the merging of smaller components? Does outflow and radiation from starburst and AGN affect the galaxy formation process? Are active, massive
forming galaxies capable of enriching the intra-cluster media with metals and thus
affect cluster evolution?
It is well established that radio galaxies at low to moderately high redshifts (z < 2)
often have bright, extended (100 − 200 kpc), emission line nebulae which are aligned
with the radio sources (see McCarthy 1993,and references therein). The morphologies,
kinematics and ionization of these nebulae have been studied extensively (e.g., van
Breugel et al. 1985; Baum, Heckman, & van Breugel 1992; McCarthy 1993; van Ojik et
al. 1997; Villar-Martı́n et al. 1999; Best, Röttgering, & Longair 2000b; Tadhunter et al.
2000; Inskip et al. 2002a). From this work one can conclude that (i) the gas is probably
leftover material from earlier galaxy merging events which involve at least one gas
rich galaxy, (ii) the kinematics of the gas along the radio source axes is disturbed due
to interaction with the radio lobes and jets, (iii) the merging events may have triggered
star formation and radio AGN activity at the galaxy centers, and (iv) the gas is ionized
in part by shocks induced by the radio sources and in part by photoionization from
the AGN (Villar-Martı́n et al. 1999; Tadhunter et al. 2000; Best, Röttgering, & Longair
2000a,b; Inskip et al. 2002a,b).
Few images exist of extended emission-line (Lyα) nebulae at much higher redshifts
(z > 3). Most known nebulae are radio-loud, being associated with radio galaxies, but
some are radio-quiet (Steidel et al. 2000; Francis et al. 2001). Nearly all these nebulae
Section 2. Sample Selection and Observations
89
were imaged with 4 m class telescopes and little morphological detail can be discerned
because of limited signal-to-noise and often moderately poor seeing. Higher quality
images are of great interest because of the potential diagnostics they may provide about
the very early stages of galaxy formation, and about starburst/AGN feedback and
chemical enrichment during this process.
For example, the existence of radio-quiet Lyα nebulae without obvious central
sources of ionization and/or outflow suggests that they may be due to accretion of
cooling gas in the gravitational fields of CDM halos (Steidel et al. 2000; Chapman et al.
2001; Fardal et al. 2001; Francis et al. 2001). Such nebulae might be the very first stage
in the formation of large galaxy, or its building blocks, a group of smaller galaxies.
The radio-loud Lyα nebulae could be the next phase, during which the central galaxy
developes a large scale starburst as a result of galaxy merging and a super-massive
black hole is activated. The ensuing outflow and ionizing radiation might then heat
and expel the accreting gas, while adding enriched material from the central starburst
to the mix, effectively stopping further galaxy growth until starburst and AGN activity
subside. Such a self regulated process of galaxy growth has been invoked as a possible explanation for the surprisingly tight SMBH/stellar bulge correlation (Silk & Rees
1998; Haiman & Rees 2001). Could it be that in the case of radio galaxies, which are
the most massive systems at any redshift, this process is further aided by radio jets and
lobes?
To investigate the nature of Lyα nebulae associated with HzRGs and exploit their
diagnostic value we have obtained deep narrow-band Lyα images of three 3 < z < 4
radio galaxies using the W.M. Keck Observatory 10 m telescopes. Here we discuss
the morphologies of these nebulae, their relationship to other pertinent imaging data
(radio, infrared, mm), and the possible implications for understanding the formation
and coevolution of massive galaxies and super-massive black holes.
Our paper is organized as follows. In Section 2 we describe the target selection,
observations and data analysis. The observational results for the individual objects
are presented in Section 3 and some physical parameters are deduced. In Section 4,
we discuss possible scenarios for the origin of the outer nebulae and the features in the
central region. We summarize our conclusions in Section 5. Throughout we adopt a flat
universe cosmology with ΩM = 0.3, ΩΛ = 0.7, and H0 = 65 km s−1 Mpc−1 . Note that at
the high redshifts of our targets a slightly different choice of cosmological parameters
can make a significant difference to the derived parameters. To emphasize this we
quote our numbers while retaining the scale factor h65 = 1. At the redshifts of our
galaxies (z = 3.4 − 3.8) and using the adopted cosmology we find look-back times
1
−1
−1
−1
<
∼12.8 h−
65 Gyr, galaxy ages ∼ 1.7 h65 Gyr, and angular scales of ∼7.6 h 65 kpc arcsec .
2 Sample Selection and Observations
The observations presented here include sensitive optical narrow-band and broadband imaging of three z > 3 radio galaxies using the Keck telescopes and previously
unpublished Hubble Space Telescope (HST) imaging of 4C 60.07. Our targets were
selected on the basis of their high redshifts and the availability of data at many other
wavelengths.
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
90
Coordinates a
Object
B2 0902+34
4C 60.07
4C 41.17
RA
(J2000)
09 05 30.11
05 12 55.17
06 50 52.14
DEC
(J2000)
34 07 55.9
60 30 51.1
41 30 30.7
R-band
Narrow-band
Gal.Ext.
mag
0.101
1.609
0.393
b
tobs
(ks)
9.6
7.2
27.6
1σ det.surf.br.
(AB mag)
29.04, 27.4
28.85, 27.3
29.85, 28.1
c
tobs
(ks)
2.4
2.0
4.0
1σ det.surf.br.
(AB mag)
29.70, 28.3
29.64, 28.1
30.08, 28.7
c
Table 1 — Summary of Keck observations
a
Position of the radio core (Carilli et al. 1994, 1997; Carilli 1995)
b
Galactic extinction at the central wavelength of the narrow-band filters
c
Formal 1σ detection limit: in the seeing disc and in a 3 00 diameter aperture respectively
2.1 Sample Selection
4C 41.17
4C 41.17 at z = 3.798 was chosen because its Lyα halo is one of the most luminous and
extended known. Previous imaging observations showed a bright elliptical-shaped
Lyα halo with a major axis of ∼ 15 00 directed along the radio axis (Chambers et al.
1990; Hippelein & Meisenheimer 1993; Adam et al. 1997; Rocca-Volmerange 1999), and
> 18 00 (Dey 1999). The
spectroscopic evidence indicated that the Lyα might extend to ∼
radio source has a multiple component, asymmetric, “double-double” FR II morphology, with the radio axis of the inner source having a position angle different from the
outer source (Chambers et al. 1990; Carilli et al. 1994). It was the first HzRG in which a
large dust mass has been detected through sub-mm observations (Dunlop et al. 1994),
implying a large SFR in the range of 2000 − 3000 M yr−1 .
The galaxy has a clumpy rest-frame UV morphology, with the brightest components aligned with the inner radio source (van Breugel et al. 1999). This light is unpolarized between λrest ∼ 1400 − 2000 Å, and thus not due to scattering from a hidden
quasar-like AGN (Dey et al. 1997). Instead, the observations show absorption line features from young hot stars which resemble those seen in z ≈ 2 − 3 star-forming galaxies and nearby starburst systems. Dey et al. derive a SFR of 140 − 1100 M yr−1 for
the central 10 kpc × 20 kpc of 4C 41.17 or 400 − 3200 M yr−1 after correction for local
extinction. These values are consistent with those derived from the sub-mm observations. In this radio source aligned star formation is thought to have been triggered by
radio jets colliding with a large, dense cloud in the forming galaxy (Dey et al. 1997; van
Breugel et al. 1999; Bicknell et al. 2000).
4C 60.07
4C 60.07 (z = 3.791; Chambers et al. 1996) is close to the Galactic plane (b II = 12 ◦ )
and suffers significant Galactic extinction (see, Table 2). Despite this unfortunate circumstance, we selected 4C 60.07 as one of our targets because its redshift is close to that
of 4C 41.17 and it is a strong sub-mm emitter and one of only three HzRGs in which
CO has been detected (Papadopoulos et al. 2000; De Breuck et al. 2002b). The large ex1
11
tent (7 00 ∼ 50h−
65 kpc) and dynamical mass (∼ 1 − 7 × 10 M ) of the cold gas are some
of the best evidence that 4C 60.07 is indeed a massive forming galaxy. A low signalto-noise image of 4C 60.07 shows a 2 00 Lyα feature aligned with the radio source. The
source displays a simple edge brightened FR II morphology (Fanaroff & Riley 1974),
Section 2. Sample Selection and Observations
91
unlike the other two objects in our sample (Carilli et al. 1994; Chambers et al. 1996).
B2 0902+34
Our third target, B2 0902+34 (z = 3.395; Lilly 1988), was selected because of its
extended, very diffuse optical morphology (Eisenhardt & Dickinson 1992), which is
very unlike that of the elongated, radio source aligned structures seen in 4C 41.17 and
4C 60.07. It is also the only HzRG for which 21 cm neutral hydrogen has been detected in absorption against the radio continuum (Uson, Bagri, & Cornwell 1991; Briggs
et al. 1993). No other evidence for cold gas or dust has been found in B2 0902+34.
No strong absorption in the Lyα emission has been detected through spectroscopy
(Martin-Mirones et al. 1995) and no significant sub-mm signatures of thermal dust
emission have been found (Archibald et al. 2001). Eisenhardt & Dickinson (1992) published a shallow Lyα image for B2 0902+34 that showed its halo to be relatively bright
but did not reveal much detail. They argued that it is a true protogalaxy, based on its
relatively flat UV-optical spectral energy distribution, indicative of a dominant population of young stars. The radio structure resembles that of quasars with a flat spectrum
nucleus, a bright knotty jet with a sharp 90 ◦ bend in the north, and a disembodied
double hotspot to the south. Carilli (1995) explained its unusual properties, inferring
that the radio axis is close to the line of sight.
2.2 Keck Imaging
2.2.1 Lyα imaging
We observed 4C 41.17, 4C 60.07 and B2 0902+34 using two custom-made, high throughput narrow-band filters. The central wavelengths and bandpasses were chosen to
cover redshifted Lyα within a velocity range of ±1500 km s−1 for each object, which
is the typical maximum width of the emission lines of HzRGs (e.g., Dey et al. 1997;
van Ojik et al. 1997). 4C 41.17 and 4C 60.07 are close in redshift and fit in the same filter, although for 4C 60.07 emission at velocities larger than 1000 km s−1 blueward from
the systemic velocity falls outside the bandpass. The transmission curve of this filter
(λc = 5839.0 Å and ∆λ = 65.0 Å) averages ∼ 84% over the bandpass. The narrow-band
filter for B2 0902+34 has λc = 5342.4 Å and a FWHM of ∆λ = 56.8 Å, with an average transmission of 73%. The fields of view through the 10 cm × 10 cm filters were
0
0
vignetted slightly, resulting in an useful sky coverage of ∼ 2 . 0 × 1 . 7.
The observations were made during the nights of UT 2000 December 27, 28 and UT
2001 February 25 using the Echelle Spectrograph and Imager (ESI; Sheinis et al. 2000)
at the Cassegrain focus of the Keck II 10 m telescope in imaging mode. The detector
used is a high-resistivity MIT-Lincoln Labs 2048 × 4096 CCD, has a plate scale of 0 “. 154
pixel−1 , and is only partly illuminated in imaging mode. Exposures were broken into
integrations of 1200 seconds and we performed 15 00 offsets between each integration.
This facilitated removal of cosmic rays and bad pixels on the CCD. The data were taken
00
00
during photometric conditions and good seeing (FWHM 0 . 6 − 0 . 8 in the co-added
images) and were reduced using standard methods in IRAF (including registering and
stacking the individual integrations). Flatfielding was done with twilight sky flats only
(as opposed to creating field flats from unregistered images) to prevent suppression of
faint diffuse emission.
92
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
The data were flux calibrated using the spectrophotometric standard stars Feige 34,
PG 1121+145, and PG 1545+035 (Massey et al. 1988). All magnitudes and colors quoted
in this paper are on the AB system and the individual calibrations agree to within approximately 10%, which we take to be the systematic photometric uncertainty. We
corrected for Galactic extinction using the E(B−V) values based upon IRAS 100µm cirrus emission maps (Schlegel, Finkbeiner, & Davis 1998) and extrapolating following
Cardelli, Clayton, & Mathis (1989). A summary of the total integration times, surface brightness limits, and Galactic extinction corrections is given in Table 2. With 1σ
surface brightness limits of NB ∼ 29 − 30 mag within an aperture of the seeing disk
(∼ 0.7 00 ) these are the deepest narrow-band exposures of Lyα nebulae ever obtained.
2.2.2 Broad-band imaging
We also obtained emission line free R-band images during the same nights to allow
continuum subtraction from the narrow-band exposures and create pure Lyα images.
For 4C 41.17 and 4C 60.07 the R-band (λ c = 6700 Å; FWHM = 1400 Å) is just redward
of Lyα and free of strong emission lines (all emission lines that fall within the filter
< 10 Å; Dey et al. 1997,Chapter 7;). In the
bandpass have observed equivalent widths ∼
case of B2 0902+34 the R-band contains the C IV λ1548,1551 doublet and He II λ1640.
The spectrum by Martin-Mirones et al. (1995) shows that C IV and He II have observed
equivalent widths of ∼ 55 Å, and ∼ 40 Å, respectively. The average total contribution
< 7% indicatof these emission lines to the total flux received in the broad-band filter is ∼
ing that they are only minor contaminants. However, the relative contribution could
depend on the region of interest. There is no easy way to correct for this, but compared
to the large equivalent width Lyα line continuum subtraction is only a small correction and does not change any morphological features of interest for this paper. These
0
0
R-band images have fields of view of 3 . 3 × 1 . 7, were broken into 400 s exposures and
were reduced following standard methods.
2.3 HST Imaging
4C 60.07 was observed during Cycle 6 with the Wide Field Planetary Camera 2 (WFPC2;
Trauger et al. 1994) on the HST. The object was placed on the PC chip, which utilizes
an 800 × 800 pixel Loral CCD as detector with a plate scale of 0 “. 0455 pixel−1 . In two
pointings 9 broad-band exposures of 2900 s each were obtained for a total exposure
time of 26.1 ks through the F702W filter, which has a central wavelength λ c = 6944.3 Å
and a FWHM of ∆λ = 1384.7 Å. For 4C 60.07 this filter includes the C IV doublet with
a combined observed equivalent width of ∼ 300 Å (Chapter 7), but is free from other
strong emission lines. After cosmic ray removal these images were co-added. The data
were severely affected by light scattering off of the Earth atmosphere. However, the
scattered light pattern was fixed with respect to the CCD. This allowed for using the
small offsets between the unregistered images of the two pointings to construct a scattering model. After subtraction of this model, the resulting image was virtually fully
corrected.
Deep (21.6 ks) F702W observations of 4C 41.17 were obtained using the PC section
of WFPC2 (for details see van Breugel et al. 1999; Bicknell et al. 2000). Observations
Section 2. Sample Selection and Observations
93
with the F702W filter are slightly contaminated by the C IV doublet, which has a combined observed equivalent width of ∼ 104 Å (Dey et al. 1997).
Eisenhardt and Dickinson observed B2 0902+34 during Cycle 4 for 21.6 ks using the
PC of WFPC2 with the filter F622W, which is centered at λc = 6189.6 Å, has a FWHM
of ∆λ = 917.1 Å, and is free of emission from the Lyα and C IV lines. These data were
obtained from the archives.
2.4 Relative Astrometry
Astrometric calibration of the Keck images was performed using the USNO-A2.0 catalog (Monet 1998). For the broad-band images 15 catalog stars were typically available
in each field, of which about 10 were unsaturated and suitable for astrometry. In all
cases this resulted in astrometry accurate to approximately 0 “. 15 rms with respect to
00
the USNO catalog. The catalog itself has a rms uncertainty of ∼ 0 . 25 with respect
to the International Celestial Reference Frame (ICRF), while the astrometric uncertain< 0 00. 2 with respect to the ICRF (for a concise discussion
ties of the radio images are ∼
of relative astrometric uncertainties see De Breuck et al. 2002,and references therein).
The narrow-band image of B2 0902+34 contains only few stars and the astrometry had
to be bootstrapped from the broad-band image which has more stars in the field. To
do this, the narrow-band image was registered relative to the R-band image to better
than a fraction of a pixel using a second order polynomial function. Similarly, the HST
images were matched to the respective Keck images resulting in relative optical astrometry better than 0 “. 1 rms, and astrometry relative to the radio maps of comparable
accuracy as for the Keck imaging.
Generally, for the 5 GHz and 8 GHz VLA radio images (Carilli et al. 1994; Carilli
1995; Carilli et al. 1997), we used the astrometry as given by the observations of the
phase calibrators. However, in the case of 4C 41.17 there was a faint optical counterpart with a well defined centroid to the serendipitous source to the SE in the 5 GHz
map (Source E in Chambers et al. 1996). We used this source to register the radio and
optical images to better than 0 “. 1. A change of only +0 “. 15 in right ascension and
+0 “. 2 in declination was required to align the images. These shifts agree with typical uncertainties in the match between the optical and radio reference frames as cited
above. Therefore, we believe that, generally, relative astrometry between the radio and
< 0 00. 35 rms, while it is better than 0 “. 1 rms in the case of 4C 41.17.
optical is better than ∼
2.5 Continuum subtraction
Pure emission line images were constructed as follows: First, the point spread functions (PSFs) of the narrow-band images were matched to the broader PSFs of the Rband images. For this we selected a suitable star close to the galaxy in both images and
calculated a convolution kernel using the IRAF task psfmatch. Subsequently, the Lyα
image was convolved with this kernel to obtain a new image with a PSF similar to the
R-band image.
The registered R-band images were then divided by a scaling factor, and subtracted
from the narrow-band images in order to remove continuum emission. This scaling
factor was determined for each radio galaxy by convolving high quality spectra (Dey
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
94
z
NB a
mag
BB−NB
mag
FLyα b
erg s−1 cm−2
log LLyα c
erg s−1
Extent
00 × 00
Size c
kpc × kpc
Mass d
108 M
B2 0902+34
3.395
19.20 ± 0.004
2.81
4.9 × 10−15
44.77
10 × 8
80 × 64
30 f v
4C 60.07
3.791
19.97 ± 0.050
3.26
8.8 × 10−15
45.14
9×7
68 × 53
3.38
9.0 × 10−15
45.15
25 × 17
190 × 130
Object
4C 41.17
3.798
18.76 ± 0.001
− 12
−1
30 f v 2
−1
130 f v 2
SFR e
M yr−1
−1
> 450 f esc
−1
> 1100 f esc
−1
> 1100 f esc
Table 2 — Properties of the Lyα nebulae
a
Determined using SExtractor (Bertin & Arnouts 1996) with automatic aperture selection and deblend
parameter optimized for detecting large structures. The uncertainties quoted are the formal error estimates given by SExtractor. The magnitudes were also determined in IRAF using a curve of growth
for the cumulative magnitude in increasing radii which resulted in magnitudes differing by at most 0.3
mag.
b
Corrected for Galactic extinction
c
Assuming a cosmology with ΩM = 0.3, ΩΛ = 0.7, and H0 = 65 km s−1 Mpc−1
d
These mass estimates are derived from M = n e m p f v V and LLyα = 4 × 10−24 n2e f v V erg s−1 (e.g., McCarthy
et al. 1990) assuming cylindrical symmetry and case B recombination. We have assumed a filling factor
f v = 10−5 , which is extremely uncertain (Heckman et al. 1982).
e
−1 L
The estimated SFR ≥ 8.12 × 10−43 f esc
Lyα is very uncertain and based on the assumption that all of
the Lyα flux is the result of Case B recombination of photoionization by a young stellar population (see
Section 3.1).
et al. 1997,Chapter 7;), after removal of the Lyα emission line, with the R-band and
narrow-band filter curves. For each galaxy this resulted in an expected ratio of continuum flux densities within both filters. These expected ratios are based only on the filter
curves, while telescope and CCD efficiencies should be taken into account as well. We
corrected for this by adjusting the scaling factor by ∼ 10%, which was the difference
between the predicted and observed ratios for the spectrophotometric standards.
3 Results
Our narrow-band images show unprecedented detail in the spectacular nebulae that
surround the radio galaxies. The nebulae extend over more than 100 kpc and display
complex morphologies such as filamentary structures, conical shapes, and distinct regions of high and low intensity. Grayscale representations (colorcycled to show both
low and high surface-brightness regions) of the pure emission line images and contour
plots of the narrow-band images (including continuum) are shown in Figures 1−8. In
the following, we discuss the morphological features of the nebulae in detail. A summary of their global properties is given in Table 2. This table also gives estimates for the
mass of the ionized gas and star formation rates. Despite being subject to considerable
uncertainties these might help provide some insight in the nature of the nebulae.
3.1 4C 41.17
The co-added continuum-subtracted narrow-band image for 4C 41.17 overlaid with a
5 GHz VLA radio map (Carilli et al. 1994) is shown in Figure 1, while Figure 2 shows the
same image overlaid with a Keck K-band image (Graham et al. 1994). Figure 3 shows
an overlay of the HST image with the narrow-band image (including continuum). The
1
−1
Lyα nebula is seen to extend over 25 00 × 17 00 (∼ 190h−
65 kpc × 130h65 kpc), nearly twice
Section 3. Results
Figure 1 — Grayscale Lyα image of
4C 41.17 with a contour representation
of the 4.9 GHz VLA map (Carilli et al.
1994) overlaid. The grayscale has been
colorcycled to show the details of the
high and low surface brightness simultaneously. The radio core is identified with a cross, and the contour levels are 0.07, 0.11, 0.4, 1.6 and 6.4 mJy
beam−1 . The arrows indicate “plumes”
of enhanced emission on both sides of
the Northern Lobe, a separate emission
line cloud with filaments extending to
the SSW and SW, a large filament, and
a Lya tongue corresponding to a region
which appears unaffected by the radio
source.
95
Plumes
Filament
Tongue
Cloud
Figure 2 — Similar to Figure 1 but with
K-band image (contours; Graham et al.
1994) overlaid. The contours are at arbitrary levels and have been chosen to
show the desired components. Some
dips in the outer halo are co-spatial
with K-band galaxies (indicated with
arrows), suggesting that these galaxies
are absorbing systems close in redshift
to 4C 41.17.
96
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
Figure 3 — HST F702W image (van
Breugel et al. 1999) of 4C 41.17 with
narrow-band image (contours; including continuum) overlaid. The contours
indicate observed surface brightness
levels at 6.7 × 10−19 × (6, 12, 25, 50, 100,
200, 400, 800) erg s−1 cm−2 arcsec−2 .
Note the group of kpc-sized clumps in
the extended diffuse emission tongue
2 00 south of the central peak, which is
not aligned with the radio axis.
the size previously seen in the images obtained with 4m class telescopes, and shows a
wealth of structure.
Central region
The bright inner region was observed and discussed in detail by Chambers et al. (1990).
It is aligned with the inner radio source and of approximately the same size, while the
radio core is located in a central dip in the Lyα emission. This has been noticed before
(Hippelein & Meisenheimer 1993; Adam et al. 1997), and Hippelein & Meisenheimer
(1993) interpret the dip as absorption by a foreground Lyman-forest cloud unrelated to
the radio galaxy. However, as proposed by Dunlop et al. (1994), it seems more likely
that the absorption is due to a dust lane orthogonal to the major axis of the galaxy and
its radio source, obscuring the center of the galaxy near the position of the radio core.
This would provide a natural explanation for both the observed sub-mm emission and
the gap in the UV continuum seen in the HST image (Fig. 3). Graham et al. (1994) also
mention that their K-band image (Fig. 2; strongly contaminated with and Hβ ) shows
evidence for a double peaked structure. They could not ascertain whether this was
also true for a line-free Ks image.
Galaxy scale
The Keck K-band and HST F702W “R”-band images (rest-frame B-band and 1500 Å
respectively) show a multi-component galaxy spread out over a region of 9 00 × 5 00
1
−1
(∼ 68h−
65 kpc × 38h65 kpc) near the center of the Lyα nebula. A Lyα tongue sticks out
just SE of the nucleus and overlaps with continuum emission seen in the Keck and
HST images. The HST image shows that this is a small group of kpc-sized clumps,
1
embedded in low surface brightness emission of ∼ 12h−
65 kpc diameter, which appears
to be unaffected by the radio source. From the total UVrest continuum for this region
van Breugel et al. (1999) derive a star formation rate of 33 M yr−1 (for the cosmological
parameters used in the present paper). For this same region we derive from our Keck
observations an integrated Lyα flux of 3 × 10−16 erg s−1 cm−2 , or LLyα = 5 × 1043 erg s−1 .
We can convert the Lyα luminosity into an estimate of the star formation rate if we
make a couple of assumptions. With the assumption that all the Lyα is due to Case B
Section 3. Results
97
−1
recombination of photoionization by stars we obtain SFR ≥ 8.12 × 10−43 f esc
LLyα . This
assumption, while unlikely to be correct for most of the halo, may be reasonable for
the undisturbed region. The estimated SFR constitutes a lower limit as it assumes that
a fraction f a = 1 of the ionizing photons will be absorbed. The typical escape fraction
found locally is f esc ≤ 0.1 (Leitherer et al. 1995). However for the z ∼ 3 Lyman break
galaxy (LBG) population (Steidel et al. 1996) f esc ∼ 0.5 − 1.0 is typically found (Steidel,
Pettini, & Adelberger 2001) and these LBG values may be more appropriate for HzRGs.
−1
From the inferred LLyα we obtain a lower limit to the star formation rate of 40 f esc
M yr−1 , rather similar to the estimate based on the UV continuum. However, both
the UV-continuum and Lyα based SFR estimates are probably lower limits since they
assume no extinction, which may very well be important given the detection of dust
in 4C 41.17.
Outer region and filamentary structure
Perhaps the most striking morphological feature of the 4C 41.17 Lyα nebula is the coneshaped structure emanating from the center of the galaxy, with long filaments and a
crescent-shaped cloud with horns. The SW outer radio lobe is located within the cone
and extends further from the core than its, much fainter, NE counterpart. It suggests
that the SW radio lobe may have encountered less dense material on its way out. This
also agrees with the emission-line and radio surface brightness asymmetry near the
center: the line emission and radio emission of the inner radio source are brighter on
the NE side. Such optical/radio asymmetry correlations have also been seen in nearby
radio galaxies (McCarthy, Spinrad, & van Breugel 1995). They are thought to be due to
local gas density asymmetries, with denser gas blocking radio jets, resulting in brighter
line and radio emission.
The filamentary/plume features of the nebula are probably caused by AGN acivity
1
and/or the radio source: the long SW filament (∼ 8 00 , 60 h−
65 kpc) is about the same size
as the distance between the nucleus and the SW hotspot, the NE lobe appears to be
embraced by two short “plumes” of enhanced Lyα emission, and the apex of the two
1
∼ 4 00 (30 h−
65 kpc) SW cloud horns projects close to the nucleus.
Another point of interest is the absence of Lyα emission near several faint K-band
companions around 4C 41.17 (Fig. 2; Graham et al. 1994). Based on their red colors
(R − Ks ∼ 4 − 5) Graham et al. concluded that these are probably multiple L ? galaxies
at the same redshift as 4C 41.17. Graham’s objects 4, 9 and 12 are all near Lyα “gaps”.
It suggests that they are dusty galaxies in the local foreground to the Lyα nebula, absorbing the Lyα photons along the line of sight. If the sizes of the gaps are larger than
the K-band objects this could be evidence for galactic envelopes (e.g., Chen, Lanzetta,
& Webb 2001). This is hard to test, given the limited signal-to-noise of the K-band
objects, but visual inspection of Figure 2 shows that they are consistent with having
1
−1
similar sizes. The scale of the Lyα absorption halos (15h −
65 kpc < D < 25h65 kpc) suggests that these represent objects which have already collapsed to “normal” galaxian
dimensions by this redshift.
The 4C 41.17 field also shows an over-density of dusty galaxies on a much larger
1
scale (∼ 2.5 0 diameter, ∼ 1 h−
65 Mpc; Ivison et al. 2000), suggesting that 4C 41.17 may
be at the center of a “proto-cluster”.
98
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
Figure 4 — Lyα image of 4C 60.07
(grayscale) with 4.7 GHz VLA map
(contours; Carilli et al. 1997) overlaid.
The radio core is indicated with a cross,
and the contour levels are 0.2, 0.4, 1.0,
2.0, 4.0 and 8.0 mJy beam−1 . A very extended NW filament and tentative evidence for a SE filament are indicated
with arrows.
Figure 5 — HST F702W image of
4C 60.07 with narrow-band image
(solid contours, not extinction corrected, including continuum) and
1.25 mm map (dashed contours; Papadopoulos et al. 2000) overlaid. The
narrow-band image has been heavily
smoothed to bring out the low surface
brightness filament and contour levels
are 6.7 × 10−19 × (5, 10, 20, 40, 80,
160, 240) erg s−1 cm−2 arcsec−2 . The
levels for the 1.25 mm emission are
at 1.2, 1.6, 2.0, and 2.5 mJy, with
σ = 0.6 mJy beam−1 , showing that the
NE component is a tentative detection
comparable in size and shape to the
restoring beam. The galaxy at the
tip of the narrow-band filament is
foreground at z = 0.891.
3.2 4C 60.07
In Figure 4 we show the continuum-subtracted image of 4C 60.07 overlaid with the
5 GHz VLA radio map of Carilli et al. (1997). Contour overlays of the narrow-band
image and 1.25 mm Plateau de Bure map (Papadopoulos et al. 2000) on top of the newly
obtained HST image are shown in Figure 5. A zoomed-in version of this HST image
with a high resolution 8 GHz VLA map is shown separately in Figure 6 to better bring
out the morphological details.
Section 3. Results
99
Figure 6 — Zoomed in grayscale representation of the HST F702W image of 4C 60.07 with 8.2 GHz VLA
map overlaid (solid contours; Carilli
et al. 1997).
The image has been
smoothed to a resolution of 0.25 00 , and
ESI R-band contours (dashed) are overlaid to better show the shape of the
galaxy. The knotty, Z-shaped structure
of the aligned restframe UV continuum is apparent, while the radio core
might be coincident with the central
gap between the two brightest emission peaks. The radio contour levels are 0.08, 0.16, 0.3, 0.6, 1.2, and
2.4 mJy beam−1 .
Central region
The radio core appears to fall into a gap in the bright central Lyα emission, and is
possibly coincident with a lack of UV continuum emission in the HST images (Figs.
5,6) similar to 4C 41.17. There is enhanced emission on either side of the nucleus,
and the Lyα emission is brightest between the nucleus and the eastern lobe which is
closest to the nucleus. This emission-line/radio asymmetry correlation is again similar
to 4C 41.17 and other radio galaxies.
Galaxy scale
The HST images shown in Figures 5 and 6 show a string of knots subtending at least
1
5 00 (∼ 38h−
65 kpc). The two brightest knots appear to be on either side of the radio core.
There is a prominent Z-shaped structure, both ends of which approximately point toward the radio lobes, making it a typical case of the radio-optical alignment effect. By
comparing the HST morphology with the Lyα emission, it seems that the Z-shaped
structure goes around the western peak of the halo (Fig. 5).
Outer region and filamentary structure
1
−1
On a larger scale the halo extends over approximately 9 00 × 7 00 (∼ 68h−
65 kpc × 53h65 kpc)
−1
00
and shows a cone shaped structure bounded on one side by a 10 (∼ 76h65 kpc) long
filament. The maximum projected sizes of the Lyα and radio structures are fairly simi1
lar, although the filament extends beyond the radio hotspot by 4 00 (30 h−
65 kpc). The tip
of the filament is co-spatial with a galaxy (Fig. 5). Although it is tempting to infer a
connection between the two, spectroscopic observations show that the galaxy is foreground at z = 0.891 (Chapter 7) and cannot be related. There is also a hint of a filament
extending out to the SE.
Interestingly, the mm emission shown in Figure 5 seems almost completely anticorrelated with the Lyα. The Lyα cone and filament appear nearly orthogonal to the
cold gas and dust. This morphology seems indicative of an outflow as it resembles luminous starburst galaxies such as M 82. In starburst galaxies this is usually interpreted
as being caused by a starburst driven superwind which blows out stellar debris and
interstellar gas and dust from the central part of the galaxy (e.g., Strickland & Stevens
2000; Ohyama et al. 2002).
100
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
Figure 7 — A continuum-subtracted
Lyα image (grayscale) of B2 0902+34
with high resolution 4.9 GHz VLA radio map (contours; Carilli 1995) overlaid. The radio core is identified with
a cross, and the contour levels are 0.2,
0.8, 1.6, 6.4, and 25.6 mJy beam−1 .
Figure 8 — Narrow-band image contours of B2 0902+34 superposed on the
HST F622W image. The contour levels are 8.4 × 10−19 × (8, 20, 40, 80, 120,
160) erg s−1 cm−2 arcsec−2 , include continuum emission, and have not been
corrected for extinction.
Section 4. Discussion
101
3.3 B2 0902+34
Figure 7 shows the Lyα image and radio map overlay of B2 0902+34. The HST image
with narrow-band contours is shown in Figure 8.
Central region
Unlike in 4C 41.17 and 4C 60.07 the central part of the Lyα and continuum emission
in B2 0902+34 is not dominated by a high surface brightness elongated, radio-aligned
structure. Instead the overall morphology appears more diffuse and bimodal, surrounding the radio core and a curved jet to the north (consistent with Carilli 1995; Pentericci et al. 1999; Fabian, Crawford, & Iwasawa 2002). Figure 8 shows that while there
is faint continuum emission to the west co-spatial with the lower luminosity Lyα peak,
there seems to be little continuum emission associated with the brightest peak. Dust is
not likely to be an important factor in obscuring any continuum on the E, because the
Lyα should then be completely extinguished in contrast to the observations and because Archibald et al. (2001) did not find strong signatures of thermal dust emission in
the sub-mm. Therefore, the source of ionization in this region remains somewhat unclear, reminiscent of the radio-quiet Lyα halos but with the important distinction that
there is a luminous AGN nearby. Given that the radio axis of the source is oriented
close to the line of sight (Carilli 1995), beaming effects are expected to be important. A
possibility is that the bright Lyα is due to scattered light from the AGN or the result
of collisional excitation. Based on the relatively flat UV-optical spectral energy distribution, Eisenhardt & Dickinson (1992) argued for the presence of a large population of
young stars. Both the inner Lyα morphology and the HST continuum might be better
understood as being due to a shocked cocoon of gas and possibly shock-induced star
formation associated with the approaching radio lobe.
Outer region and filamentary structure
The extended emission line region subtends approximately 10 00 × 8 00 on the sky (∼
1
−1
80h−
65 kpc × 64h65 kpc). Again the radio and Lyα structures are roughly comparable
in size, and the brightest radio and line emission are on the same side of the core.
However, the radio emission near bright Lyα emission is strongly polarized, unlike
4C 41.17 and 4C 60.07. This could be understood if the radio jet in B2 0902+34 is pointed
towards us. The shorter line of sight through the gaseous medium would cause less
depolarization (the Laing-Garrington effect; Garrington & Conway 1991), and at the
same time would boost the core and jet emission due to relativistic beaming.
There are few signs of large scale filamentary emission in B2 0902+34. They could be
hidden if the filaments preferentially follow the radio axis and are similarly elongated
along our line of sight.
4 Discussion
Our deep, sub-arcsecond seeing narrow-band imaging observations show previously
unknown, complex morphologies in giant Lyα nebulae around three HzRGs. Since the
nebulae are centered on masssive forming galaxies these structures may provide new
insights about this galaxy formation process and the importance of starburst/AGN
feedback. We will discuss some of the general conclusions that can be drawn from
these data. A more in-depth understanding of the nature of the nebulae will require
102
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
analysis of spectroscopic observations, which will be presented in a following paper.
For now we note that these and other spectroscopic evidence for HzRGs show that the
nebulae, at least along the major axes of the radio sources, are enriched and ionized
i.e. are not composed of pure primordial (H, He) gas and not due to Lyα scattering off
cold gas (HI). This is based on the detection of C and O along the radio sources over
many tens of kpc (e.g., Overzier et al. 2001; Maxfield et al. 2002; van Breugel et al. 2002;
Jarvis et al. 2002; Villar-Martı́n et al. 2002), including evidence for enriched material
C IV in undisturbed regions beyond radio hotspots (Maxfield et al. 2002; Villar-Martı́n
et al. 2002).
Some of the properties of radio galaxy Lyα nebulae, to the extent that they could be
studied with mostly 4 m class telescopes, have been discussed by others (e.g., Chambers et al. 1990; Eisenhardt & Dickinson 1992; Hippelein & Meisenheimer 1993; McCarthy 1993; van Ojik et al. 1996; van Ojik et al. 1997; Rocca-Volmerange 1999; Venemans et al. 2002). Here we will focus on the newly discovered features in our images:
the very large low surface brightness outer regions, the long radial filaments and coneshaped structures, and the (indirect) evidence for extended Lyα absorption. It is important to reiterate that radio-quiet Lyα nebulae also exist (Steidel et al. 2000; Francis
et al. 2001). It is not known whether the gas in these nebulae is enriched or not, and
the available images are of insufficient detail to determine whether they exhibit similar
filamentary structures as the HzRG nebulae.
A plausible scenario for explaining the origin of Lyα nebulae is one in which cold
gas from the “Dark Ages” is accreted in large Cold Dark Matter halos. In contrast to the
classical picture of gas cooling from the virial temperature, T ∼ 106 K for a typical dark
matter halo, recent theoretical models predict that most of the infalling gas may not
heat to virial temperatures but remains at T ∼ 104 K because of efficient Lyα cooling
(Fardal et al. 2001; Haiman, Spaans, & Quataert 2000). There is strong evidence that
HzRGs reside in (proto)-clusters (e.g., Pentericci et al. 2000; Venemans et al. 2002), and
a viable scenario to explain these nebulae might then be that they are signatures of
cooling flows.
As the gas accretes it will condense into stars and galaxies, and thus cooling flow
and star formation are intimately linked. The Lyα emission associated with the release
of gravitational potential energy is expected to be more spatially extended than emission related to the starburst and they are expected to be of similar magnitude in the
most luminous objects (Fardal et al. 2001). The fairly smooth and very extended outer
regions orthogonal to the radio sources suggest that these could be the remnants of
these initial, undisturbed accretion flows. The surface brightness profile in the outer
parts of the SE corner of 4C 41.17, which appears to be the least disturbed part of the
nebula, drops of with radius as approximately I(r) ∝ r−2 and would be consistent with
theoretical predictions for cooling halos (Haiman, Spaans, & Quataert 2000).
What about the radial filamentary and cone-shaped structures? How did these
form in such a scenario? Here some lessons might be learned from nearby cooling
flow clusters, Abell 1795 (Fabian et al. 2001) and NGC 1275 (Conselice, Gallagher, &
Wyse 2001; Fabian et al. 2001), starburst superwinds such as in M 82, Arp 220 (e.g.,
Heckman, Armus, & Miley 1990), and Seyfert II’s (NGC 1068; Dopita et al. 2002).
Section 4. Discussion
103
4.1 Cooling flows and radio lobes
In Abell 1795 there is a long, ∼ 80 kpc (projected size), radially directed emission-line
and X-ray filament associated with the central cD galaxy. Fabian et al. (2001) consider
several possible scenarios to explain this, including a “contrail” induced by ram pressure from the radio source. However, based on kinematic and cooling time considerations, and the fact that the cD galaxy is not quite centered at the X-ray halo, Fabian et
al. conclude that the simplest explanation would be that the filament is a cooling wake
behind the galaxy as it moves within the X-ray halo. It is possible that the bright, single filament in 4C 60.07 might be explained in this way but we consider this unlikely
because of evidence for a very low surface brightness cone-like structure (Fig. 4). The
multiple radially directed filaments in 4C 41.17 would also be inconsistent with such a
model.
In NGC 1275 numerous radial emission-line filaments are found up to ∼ 50 kpc
from the central AGN, with some tangential features at large radii. These features
overlap with a large radio halo centered on the galaxy. The fairly constant surface
brightness along the lengths of the filaments and inferred physical parameters suggest
that the filaments formed as a result of compression of the intracluster gas by the expanding radio source (Conselice, Gallagher, & Wyse 2001). The correlation between
emission-line and radio source asymmetries in each of the three radio galaxies shows
that radio jets and lobes must indeed have a significant impact on the ambient gas
emissivity and emission-line morphology. Also, the eastern radio lobe of 4C 41.17 appears to have associated “plumes” of enhanced emission straddling the lobe at both
the SW and SE, and the sizes of the radio structures are comparable to those of the
emission line gas.
We note that even if the filaments are at a significant distance from the observed radio source components, such as the SW lobe of 4C 41.17, there still is a good reason to
believe that they may be causally related, in particular if there is supporting kinematic
evidence (see 4C 29.30 for a nearby example; van Breugel et al. 1986). In the canonical picture of radio sources the hotspots and lobes are surrounded by bowshocks and
cocoons of radio quiet, shock heated gas with scale sizes wich are significantly larger
than the observed radio emission (Carilli, Perley, & Dreher 1988; Begelman & Cioffi
1989). The emission-line filaments are then located at the interface of the cocoons and
the ambient gas and are not in direct contact with the radio lobes or hotspots and can
even extend beyond radio hotspots (Figs. 1,4; Maxfield et al. 2002) if one accounts for
projection effects and the fact that the radio observations only show the highest surface
brightness regions.
4.2 Starburst superwinds
Although HzRGs may not resemble “normal” starburst systems in the strictest sense, it
seems reasonable to explore whether galactic superwinds can explain the emission-line
morphologies. Observations of low redshift starburst galaxies show weakly collimated
bipolar outflows of gas with outflow velocities of several hundred km s−1 and up to
scales of ∼ 10 kpc (Heckman, Armus, & Miley 1990). Hα emission line observations
that trace this gas show filaments and arclike structures which are possibly not too
104
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
dissimilar from the filaments that we can discern in the higher redshift Lyα nebulae.
Spectroscopic evidence for galactic outflows has been found also at high redshifts (e.g.,
Pettini et al. 1998; Pettini et al. 2001; Dawson et al. 2002) from metal absorption lines
that are blue-shifted by a few hundred to a few thousand km s−1 relative to the estimated systemic velocity of the galaxies. Recent observations of C IV absorptions systems along the lines of sight to QSOs indicate enrichment of the intra-cluster medium
(ICM) even at redshifts higher than z ∼ 5 (Rauch et al. 2001) which may have been
caused by these outflows.
For a galactic outflow powered by a superwind to occur, the star formation rate
> 0.1 M yr−1 kpc−2 (e.g.,
per unit area ΣSFR must satisfy the empirical relation ΣSFR ∼
Heckman 2001). As we have argued above, based on sub-mm detections and direct observations of stellar absorption lines, HzRGs are massive star forming systems,
with SFRs of approximately 1000−2000 M yr−1 . This implies SFR surface densities of
> 0.4 M yr−1 kpc−2 for the regions corresponding to the extent of the K-band (restΣSFR ∼
frame B) continuum. Therefore, the minimum condition for an outflow can be satisfied
easily. Similarly, the SFR surface density based on the UV flux and extent of the “undis> 33 M yr−1 /100 kpc2 ∼
> 0.3 M yr−1 kpc−2
turbed” southern region in 4C 41.17 ΣSFR ∼
would also support a galactic wind.
Models show that starburst outflows fill an over-pressured cavity of hot gas and
expand into superbubbles, but they are usually not energetic enough for the bubbles
to burst and the gas to escape from the host galaxy (Heckman 2001). This means that
for the massive HzRGs this material is likely to reside at large radii but still within the
potential well of the galaxy. Taniguchi & Shioya (2000) modelled the outflow of gas
and find the following relation for the radii of the shells:
.2
−0.2 0.6
rshell ∼ 110L0mech
,43 nH,−5 t8 kpc,
with Lmech ∼ 1043 erg s−1 M −1 the mechanical luminosity released by supernovae in
the starburst, nH,−5 the hydrogen density in units of 10−5 cm−3 and t8 the age of the
starburst in units of 108 yr. Dey et al. (1997) modelled the stellar population of 4C 41.17
with a starburst of duration 1.6 × 107 yr and an older population of stars younger than
6 × 108 yr. This gives a lower and upperlimit for t8 and Chambers et al. (1990) estimated
nH ∼ ne f v with ne the electron density and f v the volume filling factor of the clouds to
be of order 20 × 10−5 cm−3 . Applying these estimates to 4C 41.17 we obtain: 20 kpc ≤
rshell ≤ 170 kpc, resulting in a characteristic extent of the superwind shells l = 2r shell ∼
150 kpc, consistent with the sizes for the nebulae presented here.
4.3 Radiation pressure driven outflows
Recently, Dopita et al. (2002) presented a model to explain the origin and kinematics
of the narrow line regions in Seyfert Galaxies. In this model ionized gas and dust
are electrically locked together and streaming from ionization fronts around photoevaporating clouds located in the ionization cones of the AGN. A similar process might
be envisaged in HzRGs. If the dust is destroyed somewhere along the outflow, then
the gas will continue to flow outward from the cloud, but radiation pressure will no
longer dominate the dynamics. Aguirre et al. (2001) also found that dusty outflows
Section 5. Conclusions
105
might be radiation pressure driven and can reach distances of up to a few 100 kpc.
These models suggest that radiative ablation by the central source may be a significant
if not dominant contributor to outflows.
Such a scenario would explain why the filaments are all oriented radially, and why
the focal point of the tails of the SW emission line cloud in 4C 41.17 seems to be the
radio core while there is no obvious connection with the radio emission.
4.4 Obscured AGN
For all three galaxies in the sample, and for 4C 23.56 which was extensively studied
by Knopp & Chambers (1997), the radio core appears to be located in a hole or at least
a depression in the bright central part of the Lyα line emission. The best example
of this is 4C 41.17 where the UV continuum also shows a gap of approximately 0 “. 4
1
(∼ 3h−
65 kpc). Because the dips are also apparent in the UV continuum and because
there appears to be very bright emission on either side, the gap must be due to an
obscuring medium of high optical depth. MRC 1243+036 is the only HzRG with a well
studied Lyα halo for which the radio core appears associated with a peak in Lyα (van
Ojik et al. 1996). However, the central shape of MRC 1243+036 bears close resemblance
to the central cone shapes of 4C 60.07, and it is quite possible that the central region
would show two distinct components with the radio core being in between if it were
observed at higher resolution.
Hippelein & Meisenheimer (1993) were the first to notice the gap in the Lyα image
of 4C 41.17 and interpreted it as being due to a foreground absorber being part of the
Lyα forest. The strong sub-mm dust detection of 4C 41.17 led Dunlop et al. (1994) to
speculate that this gap might rather be due to a dusty disk. While this remains a viable
option, in the case of 4C 60.07 which shows a similar UV continuum and emission line
dip, sub-mm observations show that most of the dust is located “outside” the galaxy.
At least two of the three radio galaxies that we present here, show a distinct biconical shape in the brightest parts of their nebulosities. This morphology suggests that the
nucleus contributes significantly to the photoionization, and is surrounded by an obscuring torus which we observe under an angle as favored by the quasar-radio galaxy
unification scheme (Barthel 1989). This putative torus would be much larger than what
is traditionally assumed and it would be more appropriate to refer to this feature as a
dust lane. Dust lanes have been studied in nearby FR I radio galaxies (Verdoes Kleijn
et al. 1999), and appear to be orthogonal to the radio axis independent of the orientation of the galactic disk. Verdoes et al. found that these features typically range from
200 pc to 4.5 kpc, while features larger than ∼ 1 kpc show peculiar morphologies and
are possibly still settling towards stable orbits. This suggests that the large sizes seen
in HzRGs might be the result of the availability of substantial amounts of debris and
that the torus/dust lane is still forming and has not yet reached its final configuration.
5 Conclusions
We have presented a morphological study of the Lyα emission line nebulae of three
radio galaxies at redshifts z ∼ 3.4 − 3.8.
106
Chapter 6. Giant Lyα nebulae associated with high redshift radio galaxies
Our new findings are that the emission line nebulae show giant, 100 - 200 kpc
low surface brightness emission and large (up to 80 kpc) filamentary and cone-shaped
structures, which are even more extended than known previously. We have also found
indirect evidence for extended Lyα absorption due to neighboring galaxies in the local foreground to the 4C 41.17 nebula and, in all three cases, that the AGN appear
obscured by surrounding dust.
In order to explain these observations, we have presented a scenario in which primordial cooling flows in Cold Dark Matter halos form multiple stellar systems which
merge, building the stellar bulges of future massive galaxies and triggering (radioloud) SMBH activity. The radio jets, starburst superwinds and radiative ablation from
the AGN may all contribute to cause the observed radially directed filamentary structures, in at least two of the three objects studied, and may provide a feedback mechanism which regulates stellar bulge and SMBH growth. To study these processes will
require further observations of the physical properties of the gas, and numerical simulations of the effects of jets and AGN radiation on dusty, astrophysical plasmas. Some
regions of the outermost halos appear unaffected by any radio/AGN activity. This suggests that this gas must either be infalling for the first time or be a remant of previous
outflows.
Deep spectroscopy targeting non-resonant emission lines are important to constrain
the various scenarios, and determine the gas kinematics, metallicities and sources of
ionization. The observations presented here also show that filamentary structure is not
always aligned with the radio axis, and that long slit spectroscopy can easily miss large,
bright filaments. Therefore, to fully exploit narrow-band emission-line images as a tool
for studies of galaxy formation requires the use of “3-Dimensional” (2-D spatial + 1-D
spectral) imaging devices on large telescopes (van Breugel & Bland-Hawthorn 2000).
Several such devices are currently being build, including the LLNL Imaging Fourier
Transform Spectrometer (Wurtz et al.2001), and the OSIRIS imaging spectrograph for
the Spanish 10.4 m telescope GTC at La Palma (Cepa et al.2001).
Acknowledgements
It is a pleasure to thank Chris Carilli for making available the VLA radio maps, Padeli
Papadopoulos for providing us with the PdB images, Björn Heijligers for IDL tips
and tricks, and the staff at W.M. Keck Observatory for their efficient assistance. We
thank the referee, Montse Villar-Martı́n for many helpful suggestions that improved
the manuscript. The work of M.R., W.d.V., W.v.B., A.S., and M.L. was performed under
the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. W.v.B. also acknowledges NASA grants GO 5429,
5765, 5940, 6608, and 8183 in support of HzRG research with HST at LLNL. M.L.
performed part of this work at the Jet Propulsion Laboratory, California Institute of
Technology, under contract with NASA. M.D. acknowledges the support of the ANU
and the Australian Research Council (ARC) for his ARC Australian Federation Fellowship, and also under the ARC Discovery project DP0208445. This research has
made use of the USNOFS Image and Catalogue Archive operated by the United States
Section 5. Conclusions
107
Naval Observatory, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/), and
the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National
Aeronautics and Space Administration.
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Chapter 7
Metal enriched gaseous halos around
distant radio galaxies
Michiel Reuland, Wil van Breugel, Wim de Vries, Huub Röttgering, Michael Dopita, Mark
Lacy, Arjun Dey, George Miley, S.A. Stanford, Bram Venemans, Hyron Spinrad, Steve
Dawson, Daniel Stern, Astronomy and Astrophysics, submitted
We present the results of an optical and near-IR spectroscopic study of giant nebular emission line halos associated with the z > 3 radio galaxies 4C 41.17, 4C 60.07
and B2 0902+34. Previous deep narrow-band rest-frame Lyα imaging revealed
cone shapes and filamentary structures (with sizes up to 100 kpc), possibly connected to AGN and starburst feedback. In agreement with previous studies we
find that the inner high surface-brightness regions exhibit disturbed kinematics
with velocity dispersions >1000 km s−1 that seem to be closely related to the radio source. The outer regions of the halos exhibit kinematics with typical velocity
dispersions of a few hundred km s−1 , and velocity characteristics consistent with
rotation.
Spectroscopic evidence for the presence of enriched material (oxygen) throughout
the nebula of 4C 41.17 (up to a distance of ∼60 kpc along the radio-axis) is presented. The oxygen emission has a similar spatial and kinematic distribution as
the Lyα emission. We argue that this implies that the Lyα cannot be purely scattered light, and that the halo had already been enriched by a previous generation
of stars when the Universe was approximately 10% of its current age. It is also
possible that the extended oxygen has been transported from the central starburst
region aided by the radio source. We discuss various feedback processes and their
implications for galaxy formation in the context of the nature and origin of these
halos.
1 Introduction
U
the coevolution of galaxy spheroids and their central massive
black holes is one of the key issues in modern cosmology. There are at least two
reasons why high redshift radio galaxies (HzRGs; z > 3) are prime objects for studying
the processes involved. First, because they are among the most luminous and massive
known galaxies in the early Universe (e.g., De Breuck et al. 2002), they can be studied
in great detail. Secondly, we observe them early in their formation process when their
super-massive black holes (SMBHs) are highly active.
NDERSTANDING
111
112
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
Within standard Cold Dark Matter (CDM) scenarios, massive galaxies grow in hierarchical fashion through merging of smaller stellar structures and their central black
holes likely grow in similar fashion or may be primordial (Loeb 1993; Silk & Rees
1998; Kauffmann & Haehnelt 2000). In either way, outflows and radiation from active
SMBHs are expected to affect profoundly the evolution of their parent galaxies and
their environment. Models describing the formation of massive galaxies and clusters
also indicate the importance of feedback, because heating is required to prevent the
condensation of more (massive) galaxies than are observed (e.g., Benson et al. 2003).
Additionally, outflows could provide a source for the chemical enrichment of the inter
galactic medium seen at high redshifts (e.g., Rauch et al. 2001; Aguirre et al. 2001).
While there is consensus that feedback in some form must be important, there is an
ongoing debate about which manifestations dominate and the scales on which they
operate.
Most HzRGs are known to be embedded in dense gaseous environments (e.g., McCarthy 1993; Hippelein & Meisenheimer 1993; van Ojik et al. 1996; Athreya et al. 1998;
Pentericci et al. 2000; Papadopoulos et al. 2000; Wilman et al. 2004). Because of their
large spatial extents these gaseous reservoirs are excellent laboratories for studying
feedback processes at high redshift in detail.
In paper I of this series (Reuland et al. 2003) we presented narrow-band images
with unprecedented sensitivity of three HzRGs (B2 0902+34, 4C 60.07, and 4C 41.17).
These images were obtained at the Keck II 10 m telescope using custom-made, highthroughput interference filters with bandpasses centered at the redshifted Lyα line.
The observations showed very luminous (LLyα ' 1045 erg s−1 ) and extended (∼200 kpc)
emission line nebulae with spectacular filamentary structures, ionization cones, and
multiple sharply bounded regions of enhanced emission, all indicative of strong interactions as expected from the scenario painted above. We argued that the extended
Lyα nebulae may represent gas cooling in massive CDM halos, supplying material for
the continued growth of the galaxies at their center. This growth process then could
be responsible for feedback mechanisms through radio jets, supernova explosions, and
radiation pressure from the active galactic nucleus (AGN), resulting in large scale outflows. The extended X-ray halo around 4C 41.17 provides further evidence for a highly
interactive environment in such systems (Scharf et al. 2003).
While the morphologies of the nebulae were revealing some connection to the central AGN/starburst and radio source, several key questions remained. First among
these is whether the nebulae are actually ionized or, because Lyα is a resonance line,
may be due to scattering of Lyα photons produced by the central AGN/starburst off
surrounding, neutral hydrogen halos. In the latter case this could be primordial gas
from the recombination epoch. Other questions are: what is the source of ionization
and what is the chemical composition of the gas, what can we learn from the kinematics of various halo features (inner and outer regions, filaments, clouds interacting
with radio jets etc.), can outflowing gas expel metals from the deep potential wells
and enrich the intergalactic medium, and could this possibly affect accretion onto the
massive forming galaxy and growth of its central black hole? AGN / gas feedback has
been proposed as possible way to regulate galaxy and black hole growth (e.g., Silk &
Rees 1998; King 2003; Sazonov et al. 2004), which might explain the close correlation
Section 2. Observations and data analysis
Table 1 — Radio galaxies spectrographically observed. Positions of the
radio core (Carilli et al. 1994, 1997;
Carilli 1995) and the systemic redshifts
based on the He II line adopted as the
frame of reference in this paper.
113
Source
RA (J2000) DEC (J2000)
z
B2 0902+34 09 05 30.11
34 07 55.9
3.389
4C 60.07
05 12 55.17
60 30 51.1
3.789
4C 41.17
06 50 52.14
41 30 30.7
3.798
between stellar system and black hole masses (e.g., Magorrian et al. 1998; Gebhardt
et al. 2000; Ferrarese & Merritt 2000).
To help answer these questions we obtained optical and near-infrared spectra of
these galaxies with the Keck telescopes. The observations were aimed at measuring the extent, intensity, and kinematics of Lyα and the luminous non-resonance lines,
[O II] λ3727 and [O III] λ5007, at various position angles across the nebulae. This paper presents the results of these observations and considers their implications for the
nature of the line emitting gas.
The structure of the paper is as follows: The choice of targets, observations and
data analysis are described in §2. In §3 we present results for each of the targets. §4 is
a discussion of the observed kinematics and line ratios of the halos. The implications
for the origin and fate of the emission line halos are discussed in §5 and we provide a
summary of our conclusions in §6.
We adopt cosmological parameters of ΩM = 0.3, ΩΛ = 0.7, H0 = 65 km s−1 Mpc−1 ,
for which the age of the Universe is ∼ 1.7 Gyr at the redshifts (z = 3.4 − 3.8) of our
galaxies, and the angular-to-linear transformation is ∼7.6 kpc arcsec−1 .
2 Observations and data analysis
2.1 Sample Selection
The three objects selected for spectroscopic follow-up observations are shown in Table
1, with their positions and adopted redshifts. They were selected from amongst the
galaxies observed in the course of our Keck imaging program described in Paper I.
The galaxy 4C 41.17 at z = 3.8, was one of the first z > 3 galaxies to be discovered
(Chambers et al. 1990) and for many purposes serves as an archetype HzRG. Optical (Dey et al. 1997) as well as sub-mm wavelength observations (Dunlop et al. 1994)
have shown that it is a massive forming galaxy with a star formation rate of up to
several thousand M yr−1 . Recently, very extended X-ray emission was found around
4C 41.17, whose appearance follows the Lyα morphology closely (Scharf et al. 2003).
We selected 4C 60.07 (z = 3.8; Chambers et al. 1996; Röttgering et al. 1997) because
it shows both spatially and kinematically resolved CO emission (Papadopoulos et al.
2000; Greve et al. 2004). Interestingly, in Paper I it was found that the Lyα halo has a
very extended (76 kpc) filament which appears orthogonal to the major axis of the CO
and dust emission.
B2 0902+34 (z = 3.4; Lilly 1988) is thought to be a protogalaxy, dominated by young
stars (Eisenhardt & Dickinson 1992). So far, it is one of only two HzRGs for which
neutral hydrogen has been detected in absorption against the radio continuum (Uson
et al. 1991; Cody & Braun 2003).
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
114
Figure 1 — Contour representations of the narrow-band images of the Lyα emission line halos around
4C 41.17, 4C 60.07, and B2 0902+34 (left, middle, right) with the various PAs at which they were studied
overlaid. The solid and dashed lines correspond with slit positions of the optical and near-IR spectroscopy respectively and the dotted crosshairs indicate the positions of the radio cores. North is up,
East is to the left.
Object
B2 0902+34
4C 60.07
4C 41.17
UT Date
16/01/2002
07/01/2002
15/01/2002
25/02/2001
07/01/2002
24/02/2001
23/02/2001
10/12/1996
15/01/2002
03/02/1997
07/01/2002
07/01/2002
Instrument
Setup
LRIS
NIRSPEC
LRIS
ESI
NIRSPEC
LRIS
LRIS
LRISp
LRIS
LRIS
NIRSPEC
NIRSPEC
LS 600/5000
Low Disp. N5
LS 600/7500
Low Disp.
Low Disp. N6
LS 600/7500
MOS 300/5000
LS 400/8500
MOS 400/8500
LS 600/5000
Low Disp. N6
Low Disp. N7
Seeing
Slitwidth
0.9
0.9
0.7
0.8
0.9
0.7
0.6
0.9
0.7
0.9
0.9
0.9
1.5
0.76
1.5
1.0
0.76
1.5
1.0
1.0
1.0
1.0
0.76
0.76
00
00
Resolution
Å
4
14
4
13a
14
6
4
8
6
5
14
14
Wavelength coverage
Å
5150−7650
14600−17400
5450−7600
4000−9600
15600−19800
5200−6100
4300−6600
5500−9280
5300−8000
4320−6850
15600−19800
20300−25000
P.A.
◦
68.7
68.5
81.0
135.2
81.4
19.4
21.6
76.5
81.0
170.8
70.2
67.8
Table 2 — Summary of observations and instrumental setups sorted by position angle (P.A.)
a
Near the redshifted Lyα line at λ = 5825 Å; the resolution varies roughly linearly from 3 Å at 3900 Å to
40 Å at 11000 Å.
b
Spectropolarimetric observations, a detailed analysis of which was presented in Dey et al. (1997).
2.2 Optical and Near-Infrared Spectroscopy
We conducted both optical and near-IR spectroscopic observations using the LRIS, ESI,
and NIRSPEC spectrographs at the Keck telescopes. The details of the specific instrumental setups are given in Table 2 and the data reduction techniques are described
below. Figure 1 shows the slit position angles used in the program overlaid on contour
representations of the narrow-band images of the nebulae. In contrast to many previous spectroscopic studies of HzRGs, most of the slits were not placed directly along
the radio axes.
2.2.1 LRIS
Most of the optical observations were carried out using the Low-Resolution Imaging
Spectrometer (LRIS; Oke et al. 1995) at the Cassegrain focus on the 10 m Keck I Telescope. The data were collected with various instrumental setups using both the long-
t obs
s
2×1800
6×900
2×1800
3.5×1800
6×900
3×1800
4×1800
28×1200 b
3×1800
3×1800
5×900
4×900
Section 2. Observations and data analysis
115
slit mode and multi-slit masks designed to obtain spectra for ∼ 15 targets in the field simultaneously (part of a survey looking for associated galaxies in a proto-cluster; Croft
et al. 2005 in preparation). The red-sensitive LRIS-R camera was employed, which uses
00
a Tektronix 2048×2048 CCD detector with a pixel scale of 0 . 215 pixel−1 .
All of the spectroscopic reductions were performed using standard methods and
the NOAO IRAF1 package (Tody 1993). Skylines were used to improve the first order
wavelength calibration based on arc spectra to better than 0.3 Å rms. The instrumental
resolution was measured from unblended skylines. The flux calibrations were performed using observations of standard stars such as Feige 110 and Feige 34 (Massey
et al. 1988). The extended emission of 4C 41.17 filled the narrow slits of the multiobject spectroscopic program, rendering accurate sky subtraction difficult. This does
not seriously affect the kinematics or relative fluxes discussed in this paper.
2.2.2 ESI
One set of observations along the filament of 4C 60.07 was made during the night
of UT 2001 February 25, using the Echelle Spectrograph and Imager (ESI; Sheinis et al.
2000) at the Cassegrain focus of the Keck II 10 m telescope in low-dispersion mode. The
detector used is a high-resistivity MIT-Lincoln Labs 2048 × 4096 CCD with a plate scale
of 0 “. 154 pixel−1 . Exposures were broken into integrations of 1800 seconds each, one of
which had to be halved because of time constraints. We performed 6 00 offsets along the
slit between each integration to improve skysubtraction. The data reduction was done
using standard methods in IRAF. Prior to stacking the data, they were rotated over
an angle depending on the position of the object on the slit, to align the dispersion
axes. The wavelength calibration varies slightly with slit position, hence we shifted
the spectra along the dispersion axes. This first order approximation is sufficiently
accurate for the region of interest 4300Å−9200Å (900Å−1920Å rest-frame).
2.2.3 NIRSPEC
The near-infrared spectra were obtained with the 10 m Keck II Telescope using the facility Near-Infrared Spectrograph (NIRSPEC; McLean et al. 1998). We employed 0.76 00
× 42 00 slits to achieve low-resolution (R ∼ 1400−1900) spectra in wavelength ranges including their Hβ , [O II] and [O III] lines (see Table 2 for details). In this low-resolution
mode, the 1024×1024 ALADDIN InSb detector has a plate scale of 0 “. 143 pixel−1 . We
obtained sets of 900 s integrations each with ∼ 5 − 10 00 spatial offsets between exposures.
The NIRSPEC spectra need to be corrected for the slight curvature and distortions
caused by the high-throughput optics. A general correction would require rectification
onto a slit position-wavelength grid based on a wavelength solution from skylines
and coadded exposures of a standard star. However, since no continuum emission is
apparent in our 2-D spectra, we have extracted small regions close to the emission lines
1
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the
Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
116
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
only. This approach avoids interpolations because it requires just a simple rotation over
an angle that depends on the wavelength of interest.
The data were flat-fielded and corrected for cosmic rays and bad pixels in the standard fashion. In order to remove the strong near-IR skylines, a sky frame scaled to the
brightness of unsaturated sky lines near the emission line of interest was subtracted.
Subsequently, the frames were cropped, rotated, and coadded. Flux calibration was
done with stars of spectral type A0V, B3, and G4 from the 2MASS survey, and was
consistent to within 10%.
2.3 Data analysis
For the data analysis the spectra were registered in position with radio maps from the
literature (Carilli et al. 1994; Carilli 1995; Carilli et al. 1997). The zero points of the spatial scales were registered as closely as possible with the radio core. This was achieved
by identifying the core with the centroid of the continuum emission (e.g., the optical spectra of B2 0902+34, 4C 60.07, 4C 41.17). If no continuum emission was visible,
we either identified the core with the region that shows the broadest line emission or
bootstrapped the spatial solution to spectra for which the radio core could be easily
identified. These results were then checked with the narrow-band images, matching
peaks and dips in the observed intensity on the basis of our assumptions. This resulted
in a total uncertainty less than 0.5 00 rms in the relative spatial offsets.
The velocity scale used in the analysis is relative to the systemic velocity derived
from the He II λ1640 line (the adopted systemic redshifts are given in Table 1). The
kinematic information was obtained from the 2-D spectra using a program written in
IDL making use of the publicly available fitting routine MPFIT2 . The individual pixelwide traces were binned to aperture sizes corresponding with the seeing in order to
increase signal-to-noise and to ensure that the extracted spectra are not correlated. We
then determined the velocity centroid, FWHM (corrected for instrumental broadening measured from unsaturated skylines), peak and baseline fluxes by fitting a single
Gaussian curve to each trace. To fit the central multiple absorption profiles accurately
would require Gaussian fits with multiple Voigt profiles. However, since single Gaussians provide good fits to the spectra of the outer regions and it is preferable for a direct
comparison to treat the central regions similarly, we have chosen to treat all regions
consistently using the simpler approach. Therefore, the fits always represent the regions of high surface brightness emission, occasionally resulting in breaks when these
regions are embedded in more smoothly varying large scale low surface brightness
envelopes.
3 Results
Figures 2 and 3 show the extracted optical spectra of 4C 60.07 and B2 0902+34 respectively. The apertures were matched to the continuum emission from the central <3 00
regions. The rest-frame UV continuum of 4C 41.17 was discussed in detail by Dey
et al. (1997). It showed absorption lines similar to those seen in starbursting galaxies,
2
Available at http://cow.physics.wisc.edu/∼craigm/idl/fitting.html
Section 3. Results
117
Ly α
CIV HeII
CIII]
Figure 2 — Optical 1-D spectrum of 4C 60.07 extracted in a 3 00 aperture along the continuum emission.
The prominent Lyα, C IV, He II, and C III] features are indicated. A redshift z = 3.7887 ± 0.0007 is inferred based on the He II λ1640 line. The inset shows the rest-frame UV continuum. It is of insufficient
sensitivity to allow identification of reliable absorption features.
Ly α
CIV
HeII
Figure 3 — Optical 1-D spectrum of B2 0902+34 extracted in a 3 00 aperture along the continuum emission. The Lyα, C IV, and He II emission lines are indicated. A redshift z = 3.3886 ± 0.0003 is inferred
based on the He II λ1640 line. No strong absorption features can be discerned against the continuum.
118
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
indicating that the UV continuum is dominated by stars. The spectra of 4C 60.07 and
B2 0902+34 are of insufficient sensitivity to allow a similar detailed analysis. However,
since the main interest of this paper is the extended emission, we instead focus our
discussion on the nebular lines.
The 2-D optical and near-IR spectra of 4C 41.17, 4C 60.07, and B2 0902+34 centered
at the emission lines most relevant to our discussion are shown in Figures 4, 7, and
11. The vertical lines indicate the systemic redshifts of the galaxies as derived from
the He II λ1640 line. The horizontal lines indicate the positions of the outer edges of
the radio lobes and radio core projected onto the slit. It is immediately obvious that
many of the lines are very extended, and that they show strong spatial variation of
their velocity centroids and FWHMs.
An important result is that for both 4C 41.17 and 4C 60.07 the oxygen line profiles resemble those from the bright inner parts of the Lyα emission closely. Since
self-absorption cannot be important in the strong forbidden lines, this suggests that
information about the kinematic structure is preserved in the Lyα line despite it being
subject to resonance scattering. If this is true also in the outer regions, it then follows
that we can hope to obtain kinematic information using Lyα for those parts where
other lines are too faint to have been detected.
Figure 5, 6, 8, and 12, show plots of the velocity centroids, FWHMs, and normalized
flux densities of the halos as a function of distance from the radio cores for all position
angles observed. In §4.3 we will discuss the general implications of these diagrams.
First, we describe the individual sources.
3.1 Notes on individual objects
3.1.1 4C 41.17
Dey et al. (1997) have discussed the brightest part (2 00 × 1 00 ) of the extracted 1-D optical spectrum of 4C 41.17 at PA = 76 ◦ in detail. They determined a redshift z =
3.79786 ± 0.00024 based on the He II line. Furthermore they found evidence for stellar absorption lines, and low polarization indicating that a young stellar population
contributes significantly to the rest-frame UV continuum emission.
The 2-D spectra along the radio axis (PA 76 ◦ ; Fig. 4) show that the Lyα, [O II]
λ3727, and [O III] λ5007 emission line regions are very extended (over ∼20 00 , 10 00 , and
6 00 , respectively). The Hβ line is much fainter, but also appears extended. As has
been noted above, the velocity structure of the Lyα emission resembles that of oxygen
closely despite the resonant broadening of Lyα. The Lyα and [O III] lines both consist
of two separate components straddling the radio core with peak fluxes separated by
about 3 00 . Careful inspection reveals that these components are present also in the
[O II] line, with the red part of the western component missing due to a skyline near
λ = 17880 Å. The dip in between the emission peaks is expected since the narrow-band
Lyα and radio image overlays showed the core to be highly obscured (see Paper I).
This suggests that the lack of Lyα near the core is caused by a dusty medium, and not
only by large columns of neutral hydrogen.
The image overlays indicated that the position of the radio core is offset by 0.5 00
to the NE from the center of the dip in the line emission. Figure 4 provides further
Section 3. Results
E
Lyα
119
PA=76
W
E
W
E
[OII]
PA=70
[OIII]
PA=77
W
Ηβ
PA=77
E
W
Figure 4 — Grayscale representations of the 2-D spectra of 4C 41.17 centered at the Lyα, [O II] λ3727,
Hβ , and [O III] λ5007 lines. The velocities are indicated relative to the systemic redshift determined
from the He II line. The zero points of the spatial scales correspond with the position of the radio core as
identified with the broad line and continuum emission. Note that the peaks of the line emission corresponding with the position of radio knot B2 in Carilli et al. (1994) are 1 00 E of the nucleus. Furthermore
it can be seen (most easily in the [O III] λ5007 spectrum) that there are 0.5 00 offsets between the broad
lines and the center of the depression. The [O II] spectrum is rich in skylines. These have been masked
to better show the structure of the [O II] line emission. The dashed lines correspond to the positions of
the outer radio lobes and radio core projected onto the slit. The vertical line represents the the adopted
systemic velocity of the galaxy. The thick lines in the Lyα panel indicate the two velocity regimes and
the projected spatial positions for which CO emission has been detected (De Breuck et al. 2004). The
position angles of the slits are indicated in the top right corner.
120
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
Figure 5 — Relative velocities (solid lines), velocity dispersion (bars), and normalized surface brightness profiles (dashed lines) as determined from spectra with slit positions parallel to the radio axis of
4C 41.17. Top panels: Lyα for PA=76 ◦ (left), Lyα for PA=81 ◦ (right). Bottom panels: [O II] for PA=70 ◦
(left), and [O III] for PA=68 ◦ (right). The spatial zero points correspond with the position of the radio
core and the dotted lines represent the position of the radio lobes projected on the slit. Bar and symbol
size indicate the 1σ uncertainties on the measurements. The left axis indicates the scale for the relative
velocities, the right axis for the FWHMs. In the central region of 4C 41.17 the formal errorbars on the
fitted FWHMs are very small. However they are not representative of the diffuse extended halo. Note
that the fits to the [O II] emission are affected by the strong skylines and that the slit with PA=81 ◦ did
not intersect the radio core.
Section 3. Results
121
Figure 6 — Similar to Fig. 5 but for Lyα emission along the slits that are approximately perpendicular
to the radio axis of 4C 41.17. PA=19 ◦ (top right), PA=22 ◦ (top left), and PA=171 ◦ (bottom).
122
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
evidence for this without relying on relative astrometry: in the spectra, the position of
< 0.9 00 ) tail of broad (1000 km s−1 FWHM)
the radio core is identified with a narrow (∼
[O III] λ5007 emission starting at λ ∼ 23930 Å, which appears to be symmetrically distributed around the systemic velocity.
Kinematics
Figure 5 shows that for Lyα the central regions show high (∼ 800 km s−1 ) velocity
dispersions which extend along the radio axis to the outer radio lobes. The velocity dispersions inferred from [O III] are similarly high (∼ 500 km s−1 ) in the region for which
they could be determined. Beyond the radio lobes there is a break and the Lyα kinematics become more quiescent (∼ 400 km s−1 ) and the velocity centroids change from
being blue-ward shifted (−500 to −700 km s−1 along the (south-)western lobe) to near
systemic. Perpendicular to the radio axis (see Fig. 6) the kinematics follow a smooth
gradient with the high FWHMs more limited to the central region of ∼3 00 (∼20 kpc)
wide that is identified by the break in peak surface brightness distribution.
A very important result from the near-IR spectroscopy is that there is [O II] emission
in the velocity regime −200 to −1000 km s−1 extending from the nucleus to 7-8 00 west
of the nucleus, whereas the [O III] emission is much more centrally concentrated. We
will discuss this further in §4.1 and §4.2.
If one neglects possible radiative transfer and obscuration effects in the Lyα emission (as seems justified by the similarity to the oxygen emission), there appears to be a
velocity shear in the outer parts (undisturbed by the radio source) along the radio axis.
The kinematic profiles with slit positions perpendicular to the radio axis (Fig. 6) show
a fairly symmetric velocity distribution.
Interestingly, the velocity structure of the bulk of the emission line gas is predominantly redward shifted in the O, Hβ , and Lyα lines with respect to the systemic velocity
inferred from the He II emission line. We will discuss this in §4.4.
3.1.2 4C 60.07
Figure 2 shows the 1-D extraction of the ESI spectrum of 4C 60.07 (PA = 135 ◦ ) in a 3 00
aperture to include all continuum emission. Based on the He II line at 7855.7 ± 1.1 Å
we infer a redshift z = 3.7887 ± 0.0007, in agreement with Röttgering et al. (1997).
The 2-D spectra are presented in Figure 7. The top panels show the Lyα and C IV
emission lines along PA = 135 ◦ . The two bottom panels show that, as was the case
for 4C 41.17, the morphologies of the Lyα and [O II] emission line velocity structures
(PA = 81 ◦ ) are remarkably similar, providing further justification for the use of Lyα as
a tracer of the dynamics of these systems. There is one significant difference between
[O II] and Lyα however: the [O II] line exhibits two emissions peaks at both sides of the
location of the radio core, while the Lyα emission near the systemic velocity shows a
smooth gradient from one side to the other.
The 2-D Lyα emission line for PA = 135 ◦ is presented at a different stretch in Figure
9, emphasizing the difference between high and low surface brightness regions. Interesting structure is apparent: crescent shaped clouds surround a ‘gap’ at the position of
the radio core near the systemic velocity of the galaxy. We will discuss this further in
§4.2.
Section 3. Results
SE
Lyα
123
PA=135
NW
E
W
SE
CIV
PA=135
[OII]
PA=81
NW
Lyα
PA=81
E
W
Figure 7 — Similar to Fig. 4. Grayscale representations of the 2-D spectra of the emission line halo
around 4C 60.07. The top left and top right panel respectively show Lyα and C IV emission along the
filament with PA = 135 ◦ . The bottom left and right panel show Lyα and [O II] λ3727 along the radio
axis with PA = 81 ◦ and have a different scale to bring out more details. Therefore, in the two bottom
panels the projection of the western radio lobe (at 6 00 ) falls outside the figure. Two CO components have
been detected for this galaxy (Papadopoulos et al. 2000; Greve et al. 2004). The thick lines indicate their
velocities and positions as projected on the slit.
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Chapter 7. Metal enriched gaseous halos around distant radio galaxies
Figure 8 — Similar to Fig. 5 but for 4C 60.07. Lyα (top left) and [O II] (top right) emission along PA=81 ◦
and Lyα emission along PA=135 ◦ (bottom). The datapoints representing the fitted FWHMs for the center
region fall outside the plot. See the text for details.
Section 3. Results
Figure 9 — Similar to the Lyα emission line at PA = 135 ◦ in Figure 7. The
contrast has been adjusted to bring out
crescent shaped arcs surrounding a depression in the emission that is identified with the location of the radio core
and unobscured optical nucleus.
125
SE
Lyα
NW
Kinematics
Figure 8 shows that both the Lyα and [O II] lines at PA = 81 ◦ have very high velocity
FWHMs of ∼ 1600 − 1700 km s−1 , rather large for HzRGs. These are however smaller
than for the spectrum at PA = 135 ◦ which shows a FWHM of 2600 km s−1 near location
of the ‘Lyα gap’ discussed before.
For the central (+2 00 to −3 00 ) region of Lyα and C IV at PA=135 ◦ , emission at the
eastern side seems preferentially blue-shifted by ∼100–200 km s−1 . In contrast, most of
emission at the western side is redshifted by a comparable amount. Lyα and [O II] at
PA=81 ◦ show similar structure.
The Lyα Filament
The most striking feature of the emission line nebula surrounding 4C 60.07 is the
extended Lyα filament. Imaging showed this filament to have approximately constant
surface brightness, sharply bounded on the NE side, and more “fluffy” on the SW side.
The tip of the filament is co-spatial with a small galaxy suggesting a possible physical
connection. In Figure 10 we present the full 2-D spectrum of this filament obtained
with ESI.
Figures 8 and 9 show that the filament is offset from the systemic velocity of the
radio source with evidence for a velocity gradient across the filament. The gas seems
to be moving at a relative velocity of −100 km s−1 near the tip, with the absolute velocity offset gradually increasing to −400 km s−1 close to the galaxy. The central wavelength fit by a single Gaussian to the coadded filament yields λ ∼ 5818.1 ± 0.3 Å or
−175 km s−1 relative to a systemic velocity at z = 3.789 with a FWHM of 20 ± 1 Å corresponding to a deconvolved velocity dispersion of ∼ 700 km s−1 .
Both spectroscopy and imaging show the filament to be of approximately constant surface brightness with well defined spatial limits. However, the apparent sharp
126
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
Figure 10 — 2-D (Top panel) and 1-D spectrum (Middle panel) of the galaxy at the extremity of the
Lyα filament of 4C 60.07. No strong redshift constraints could be obtained based on this spectrum. A
possibility is that the galaxy is at z ∼ 0.9, close to that of brighter foreground galaxy (z = 0.871) that
was also on the slit and is plotted in the bottom panel. The circles indicate emission lines which could
be identified with redshifted [O II] λ3727. It is also possible that the galaxy is at a redshift similar to
4C 60.07. See the text for details.
Section 3. Results
127
boundary of the filament could be due to the filter bandpass whose response cuts
off sharply at the blueward limit of the wavelength observed (the lower limit of the
narrow-band filter is 5806 Å). If the filament is part of larger scale structure with coherent motion extending blueward this would fall outside the filter, explaining the
seemingly sharp boundary, and the observed velocity structure would be a function of
the position selected.
Galaxy at the tip of the filament
As shown in Paper I there is a galaxy co-spatial with the observed extremity of
the Lyα filament. Figure 10 shows the 2-D and 1-D spectra of this galaxy. The spectra show faint continuum emission which extends blueward from the Lyα filament, a
possible faint emission line near 7050 Å and a steady rise of the continuum longward
of approximately 7200 Å. Unfortunately, strong redshift constraints cannot be obtained
from these spectra. One possibility is that the redshift is very similar to that of a nearby
brighter galaxy which was also on the slit. The brighter galaxy shows a line and break
near 7000Å. The bright emission line is best identified with [O II] λ3727 at z = 0.871.
If the possible emission line near 7050 Å is also identified with [O II] λ3727 and/or the
rise in the continuum emission with a Balmer/4000 Å break, then a redshift of z ∼ 0.9
is inferred for the galaxy at the tip of the filament, possibly suggesting that 4C 60.07 is
observed through a foreground group.
Alternatively, it is possible that the galaxy at the tip of the filament is actually at a
redshift similar to 4C 60.07. The following arguments are in favor of this interpretation:
(i) The location. Although a chance superposition is possible, it seems that the Lyα
filament ends exactly at the position of this galaxy suggesting a physical connection.
(ii) The galaxy has colors which are redder than for 4C 60.07 and the brighter galaxy,
as expected for distant galaxies without an AGN. (iii) In the CO map of Papadopoulos
et al. (2000) there is a hint of CO line emission at the location of the galaxy. If correct,
then this galaxy would most likely be a dusty starburst galaxy without strong emission
lines, at the redshift of 4C 60.07.
3.1.3 B2 0902+34
Our optical spectrum of B2 0902+34 shown is the most sensitive exposure at a fixed
PA yet obtained (e.g., Lilly 1988; Martin-Mirones et al. 1995). The 1-D spectrum (Fig.
3) shows that the integrated continuum emission (3 00 aperture) has an almost constant
surface brightness and a faint break blueward of Lyα. We consider the He II line at
7199.4 ±0.5 Å, extracted in a 3 00 wide aperture centered at the radio core, to be most
representative of the systemic velocity of the galaxy. Accordingly, we infer a redshift
z = 3.3886 ± 0.0003. This value for the redshift is slightly less than reported previously
(Lilly (1988) and Martin-Mirones et al. (1995) inferred z = 3.395 and z = 3.391 respectively) but entirely consistent with the redshift estimate from fitting the Lyα line with
a Gaussian envelope and strong Voigt absorption profile in the same aperture.
From the 2-D spectra we detect Lyα emission over an extent of ∼ 10 00 , having a
complex multi-component spatial and velocity structure (c.f., Fig 11). The 2-D structures of the C IV and He II lines differ significantly from the overall shape of the Lyα.
Inferred redshifts for B2 0902+34 depend on both the emission line and aperture used.
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
128
ENE
Lyα
PA=69
WSW
ENE
WSW
ENE
CIV
PA=69
[OII]
PA=69
WSW
HeII
PA=69
ENE
WSW
Figure 11 — Similar to Fig. 4, but for B2 0902+34. The top left panel shows the line emission profile for
Lyα, top right C IV, bottom left He II, and bottom right [O II].
Section 3. Results
129
Figure 12 — Similar to Fig. 5 but for B2 0902+34. Lyα (left) and [O II] (right) both along PA=69 ◦ .
This illustrates the difficulty of assigning single redshifts to such complex systems and
explains the small discrepancies with redshifts from the literature discussed above.
The [O II] spectrum is too shallow to allow a detailed comparison with the Lyα
emission, but is consistent with [O II] being similarly distributed.
Given that B2 0902+34 shows associated H I absorption against the radio continuum
it is important to see whether this is present also in the Lyα profile, since no signatures
of this have been seen previously in the optical. Associated OH emission has been
reported also, but follow-up observations have failed to confirm this (Cody & Braun
2003).
The spectrum with an aperture encompassing all Lyα emission shows an absorption feature blueward of the Lyα line at λ = 5331.58 (z = 3.3857), which can be fitted
with a Voigt profile with a column density of 1.8 × 1014 cm−2 and Doppler parameter b = 195 ± 11 km s−1 . This however cannot be the same gas that causes absorption
against the radio continuum, since Briggs et al. (1993) and Cody & Braun (2003) infer
a redshift zabs = 3.3962 with NH = 3 × 1021 cm−2 and FWHM = 120 km s−1 for the absorbing gas. At this redshift a weak feature exists can that can be fitted with a small
additional Voigt profile at λ = 5343.0 ± 0.3 with Doppler parameter b = 104 ± 76 km s −1
and column density NH = 1.7 × 1013 cm−2 . This is unlikely to be the same gas however, because the inferred column density is eight orders of magnitude smaller than
estimated from the radio observations. This second ‘absorption’ feature is more easily
explained as an artefact of representing the triangular shape of the Lyα emission and
the low surface brightness emission with a single spectral profile.
Kinematics
None of the emission lines show strong indications for rotation or any ordered motion. The kinematics of the Lyα emission is very interesting however, as it seems to consist of two components. One component being the typical triangular shape resulting
from spatially extended absorption that is blue-shifted relative to the systemic velocity
(c.f., Dey 1999). The other component is the Lyα emission near the position of the
radio core, but at a relative velocity of −500 km s−1 . The C IV emission is preferentially
blue-shifted with respect to the systemic velocity of the galaxy. It covers a velocity
130
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
range of approximately −1200 km s−1 to +400 km s−1 . The centroid is at −400 km s−1 ,
close to the centroid (−500 km s−1 ) of the blue-shifted Lyα near the postulated position
of the radio core.
4 Discussion
We now discuss the implications of our observations. We address the following questions: What processes are ionizing these halos? Can Lyα be used as a reliable tracer of
the velocity field of the gas? What are the small scale and large scale kinematics of the
halos and outflows? What are the metalicities of the gas and the estimated masses of
the galaxies? Can outflows from the HzRGs escape the deep potential wells and enrich
the IGM? What is the impact of these outflows on the radio galaxies themselves?
4.1 Ionizing source
In studying the nature of the ionized gas halos one of the most important questions
concerns the nature of the mechanism that is responsible for ionizing the extended
emission line nebulae. The most likely candidates are: photoionization by radiation
from an AGN or stars, shock heating, and shocks with precursors that photoionize
the region ahead of the shock by radiation from the gas (e.g., Allen et al. 1998, and
references therein).
Studies of z ∼ 1 radio galaxies have shown that the dominant ionization mechanism
may depend on the evolutionary state of the radio source (e.g., Best et al. 2000; Inskip
et al. 2002). Generally, it was found that the emission line gas of small (i.e. young)
sources is shock ionized, but as the radio source expands beyond the host galaxy, interactions with the gas decrease and photoionization by the AGN takes over. As De
Breuck et al. (2000) noted, for HzRGs the situation is likely to be more complex because
of ongoing merging of galaxies and their halos, jet-induced star formation, entrainment
by the radio source and outflows.
The total Lyα luminosities LLyα ' 1045 erg s−1 (Paper I) can be explained both by
(i) an embedded QSO with a power-law spectrum with index α = −1.4 ( f ν ∼ ν α ) and
combined ionizing and mechanical energy flux of at least 1 × 1046 erg s−1 and (ii) by a
> 1000 M yr−1 . The required star formation rate is
starburst with a star formation rate ∼
remarkably close to what is deduced from rest-frame far-IR observations (Dunlop et al.
1994; Stevens et al. 2003). However, based on detailed modeling Bicknell et al. (2000)
argue that the dominating ionizing mechanism is shock excitation through jet-cloud
interactions with fast ∼ 1000 km s−1 shocks. Scharf et al. (2003) proposed that X-ray
emission could be responsible for ionizing the outermost parts of the halo.
Allen et al. (1998) showed that a combination of rest-frame UV (C IV λ1549/He II
λ1640) and rest-frame optical ([O III] λ5007/Hβ ) line fluxes can help to separate pure
photoionization from shock dominated mechanisms. This diagram is relatively insensitive to the effects of dust extinction as the ratios are determined from lines close in
wavelength. Iwamuro et al. (2003) conducted a rest-frame UV-optical emission line
study of 15 radio galaxies with 2 < z < 2.6. They found that there is a range in observed line-ratios suggesting that some objects are best explained with photoionization of low metalicity gas while others are consistent with the shock+precursor model.
Section 4. Discussion
131
Figure 13 — Top: Normalized surface brightness profiles of the [O II] (solid line) and Lyα (dashed line)
emission along the inner radio axis and south-west filament of 4C 41.17 in the velocity range +200 to
+700 km s−1 , showing extended [O II]. The spatial zero-point corresponds to the position of the radio
core. Bottom: Similar to top panel but for the velocity range −200 to −1000 km s−1 . The [O II] emission is
detected out to ∼ 8 00 (∼ 60 kpc) west of the nucleus, where it is blue-shifted by ∼ −600 km s−1 relative to
the He II line. The projected distance of the south-west and north-east radio lobes along the slit direction
are indicated (dotted lines). The +200 to −200 km s−1 range for [O II] is affected by near-infrared sky
lines and is not shown.
Carson et al. (2001) and Maxfield et al. (2002) found evidence for changing line ratios
within sources, suggesting that the dominant ionization process depends on the region
of interest. The different distributions of the [O II] and [O III] emission for 4C 41.17
also suggest a change in ionizing mechanism. Therefore, we discuss the central and
extended regions separately.
4.1.1 Central region
Based on previously published values of [O III]/Hβ ∼ 3.4 and 2.8 for 4C 41.17 and
B2 0902+34 respectively (Eales & Rawlings 1993) it seemed that pure photoionization
models could be ruled out. However the Hβ detections were marginal, and using the
total integrated line fluxes for 4C 41.17 from the present study we derive ratios of [O III]
λ5007/Hβ ∼9.6 for the central region, and [O III] λ5007/Hβ ∼ 11.8 for the spatially
integrated spectrum. These are much larger than the values reported by Eales and
Rawlings and suggest either a pure photoionization or a shock+precursor model.
4.1.2 Extended region along the radio axis
The degree of ionization in the extended regions, at large distances from the AGN, appears different from that near the galaxy centers. As discussed in §3.1.1, for 4C 41.17,
[O II] emission was detected as far as ∼ 60 kpc from the nucleus. Figure 13 shows
the velocity integrated relative intensities of Lyα and [O II] as a function of distance
from the nucleus for a red-shifted (+200 to +700 km s−1 ) and a blue-shifted (−200 to
132
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
−1000 km s−1 ) velocity regime. The red-shifted [O II] follows the Lyα closely, while
the blue-shifted [O II] shows a relative enhancement over the range 3–8 00 W of the
nucleus. In this regime no evidence for [O III] is found (Fig. 4), whereas the [O III]
emission in the nuclear region is much brighter than the [O II]. The [O III]/[O II] ratio
is a tracer of the ionization parameter. It changes from ∼3–4 in the center to < 0.5
in the outer regions. Naively, one could interpret this as a natural result of softening
of the ionizing spectrum with increasing distance from the central source. However,
representing the entire emission line region with a single H II region is not realistic.
The halo is better described with a multi-phase medium, in which cool, dense clouds
are in pressure equilibrium with a hot surrounding medium. In this scenario the extended [O II] emission depends on the detailed ionization balances of many individual
clouds. Explaining the observed spectrum with photoionization from a central region
would require fine tuning the density distribution of these clouds. The extended [O II]
emission is therefore much better described as the result of shocks related to the radio source interacting locally with the emission line clouds. This is sensible since the
emission is located along the radio-axis and both the Lyα and [O II] are blue-shifted by
∼ 600 km s−1 , likely related to the expansion of the radio cocoon.
4.2 Radiative transport: to scatter or not to scatter
As mentioned already in §3 one important result of the near-IR spectroscopy is that
for both 4C 41.17 and 4C 60.07 (and possibly also B2 0902+34, but the spectrum is not
as sensitive) the velocity structures of Lyα resembles those of oxygen closely. This is
unexpected since Lyα is subject to resonant broadening in the damping wings. However, in the previous section we have argued that the extended filaments along the
radio axis are actually locally ionized. It may be that these extended parts are not sufficiently dense for significant line broadening of the Lyα emission to occur. The R-band
image shown in Paper I is free from strong emission lines. Yet, it shows a close correlation with the narrow-band Lyα image. This indicates that morphological information
about the ionizing continuum is contained in the narrow-band image despite the expected resonant scattering of the Lyα. Together, we believe this justifies our use of Lyα
as a tracer of the underlying kinematics in those regions where other emission lines are
too faint to have been detected.
In contrast to what was found for the extended regions, resonant scattering appears
to play a significant role in the central regions. Figure 9 shows that in the Lyα spectrum
of 4C 60.07 at PA = 135 ◦ there is a ‘gap’ at the postion of the radio core. A comparable ‘hole’ in Lyα emission near the systemic velocity of a galaxy has been found for
SSA-22 Lyα Blob 1 (Bower et al. 2004) with the SAURON integral field spectrograph.
It seems reasonable to attribute these velocity profiles to complex radiative transport
effects of the resonantly scattered Lyα emission in dense (dusty) media (e.g., Neufeld
1990; Ahn 2004) since both sources are strong submillimeter (rest-frame far-IR) emitters (Papadopoulos et al. 2000; Chapman et al. 2001). The emission line profile across
the Lyα gap can be fitted with two Gaussian components with central wavelenghts
of 5839.9 ± 0.5 Å and 5806.8 ± 0.4 Å and FWHMs of 12.8 ± 0.5 Å and 10.6 ± 0.3 Å respectively. This double peaked structure is very similar to the one inferred for static
> 10
halos with coherent scattering and complete redistribution for an optical depth τ ∼
Section 4. Discussion
133
(c.f., Fig. 1 in Meinköhn & Richling 2002). Figure 8 shows that Lyα and [O II] at PA
= 81 ◦ both have very high velocity FWHMs of ∼ 1600 − 1700 km s−1 . The spectrum at
PA = 135 ◦ shows the ‘Lyα gap’ and has a much broader FWHM of 2600 km s−1 .
It seems that the high H I densities near the cores result in resonant scattering to
completely dominate the observed velocity profiles. In constrast, the similarity between the [O II] and Lyα profiles at PA = 81 ◦ suggests that outside the central regions
Lyα can be used as a tracer of the kinematics of the gas.
4.3 Kinematics
4.3.1 General kinematic structures
The general characteristics of the spectra presented in this paper are similar to what has
been reported on previous observations of emission line regions around radio galaxies
in the literature (e.g., van Ojik et al. 1996; Dey 1999; Villar-Martı́n et al. 2003). They
show clear evidence for a distinction between disturbed and more quiescent regions.
The inner regions have high surface brightnesses, are characterized by large velocity
dispersions (FWHM ∼ 1500 km s−1 ) which are likely the result of shocks at the edge
of the expanding radio source, and seem to be embedded in low surface brightness
regions with FWHMs of order ∼ 500 km s−1 .
Except for the high velocity tails originating close to the nucleus the oxygen lines
have typical FWHMs ∼600 km s−1 , which is narrower than seen in the Lyα lines. This
is probably because the oxygen lines better reflect the real motions of the gas, whereas
the Lyα lines may have been broadened by resonant scattering.
In addition to evidence for interactions with the radio sources there is evidence for
velocity shears in the extended regions of both 4C 41.17 and 4C 60.07. Moreover, Fig.
6 shows that for 4C 41.17 the velocity stucture along PA ∼ 20 ◦ (at an angle of ∼ 55 ◦ to
the radio axis) has a symmetric distribution, as expected for rotation.
For B2 0902+34 none of the emission lines show strong indications for rotation or
any ordered motion.
4.3.2 Outflows
Evidence was found also for large scale outflows. Below we discuss the findings for
each galaxy in detail.
4C 41.17
The velocity structure of the bulk of the emission line gas appears predominantly
redward shifted in the O, Hβ , and Lyα lines. A similar velocity structure but shifted
bluewards in its entirety is seen in the extended CO J=4−3 emission (De Breuck et al.
2004), suggesting that the AGN may be offset from the systemic redshift of the galaxy.
The central CO component is situated at a relative velocity of −125 km s−1 . This is
similar to the relative velocity (−135 ± 45 km s−1 ) of the absorption seen in the low
ionization species of [S II], C II, and [O I] (Dey et al. 1997).
The off-center CO component coincides with the position of the Lyα gap between
the ‘cloud’ and the galaxy as discussed in Paper I, suggesting that it is absorbing
134
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
the Lyα emission at this location. Interestingly, it has a velocity of −550 km s−1 relative to systemic, surprisingly close to the measured velocity offset (approximately
−600 km s−1 ) of both the Lyα and the [O II] emission along the Western filament.
4C 60.07
The most striking feature of the emission line nebula associated with 4C 60.07 is
the extended Lyα filament. Imaging showed this filament to have approximately constant surface brightness, to be sharply bounded on the NE side, and more “fluffy” on
the SW side. As was discussed in Paper I the overal morphology is suggestive of a
cone/superwind like structure. The galaxy at the tip of the filament would then be
a chance superposition. If, however, the galaxy at the tip of the filament is at a redshift comparable to 4C 60.07, another explanation may be more likely. In this case the
filament is best explained with a tidal tail. The galaxy may have passed through the
gaseous halo of 4C 60.07 and due to ram pressure is being stripped of its gas. Such tails
are seen also in cosmological simulations by for example Springel & Hernquist (2003)
and could explain the lack of emission lines since only little fuel for star formation
and AGN activity would remain. This would also explain naturally why the velocity offset between the filament and 4C 60.07 is largest near the galaxy. This is where
the infalling galaxy reached its largest relative velocity, and it has been decelerating
since and is likely to reach its turn-around point soon. For an average relative speed of
∼ 250 km s−1 the encounter must have happened approximately 300 Myr ago. This is
comparable to the typical timescales of starbusts, but much longer than the radiative
timescale (∼ 105 yr; Osterbrock 1989) of Lyα emission for conditions typical of the ISM.
This implies that the gas in the filament must have been recently ionized. What can be
the source of the ionization? One possibility is that stars which may have formed in
this tidal tail, similar to what is seen in the UV observations with GALEX (Martin et al.
2003) of the Antennae, provide the ionizing radiation. Another possibility is that the
galaxy is moving through a dense inter cluster medium, which is gravitationally focussed by the passage of the galaxy and is then cooling in its wake. This latter scenario
was proposed by Fabian et al. (2001) to explain an 80 kpc filament seen in Abell 1795.
B2 0902+34
The velocity structure of the Lyα emission around B2 0902+34 seems to consist of
two components. One component being the typical triangular shape resulting from
spatially extended absorption that is blue-shifted relative to the systemic velocity (e.g.,
Dey 1999). The other component is the Lyα emission near the position of the radio
core, but at a relative velocity of −500 km s−1 . One explanation for this blue-shifted
component is that the H I density in this region is lower than elsewhere, such that the
Lyα is not completely absorbed. While it is certainly likely that the gas on the Eastern
side of the galaxy is denser, as this is where imaging showed it to be brightest (stronger
interaction with the radio source), this may form only part of the explanation.
The fact that only the blue-shifted component appears to have associated C IV λ1549
emission is surprising and argues against a simple density effect. A more interesting
possibility is that this is a sign of a metal rich outflow from the central region, the result
of a direct interaction between the peculiar radio source (see Carilli 1995, for details)
Section 4. Discussion
135
with the emission line gas, or possibly a sign of a merger between a metal-poor and
metal-rich component (c.f. Overzier et al. 2001). A merger of two gaseous halos is also
a viable option to explain the global structure of the emission line nebula which shows
two bright areas. One of these areas did not show associated continuum emission,
possibly because the stars in merging systems experience less drag than the gas and
are moving faster towards the center of mass. However, it would be difficult for this
latter scenario to explain how the smaller component could be in a more advanced
evolutionary state. The most likely explanation therefore is that the blue-shifted Lyα
and C IV emission is related to a metal enriched outflow, possibly driven by the passage
of the radio jet.
4.4 A comparison with CO observations
It is interesting to relate the ionized gaseous halos of 4C 41.17 and 4C 60.07 to recently
discovered molecular gas associated with these objects.
For 4C 60.07, observations with mm-interferometers of the redshifted CO J=4−3
transition have resulted in the discovery of two kinematically and spatially separate
gaseous reservoirs (Papadopoulos et al. 2000). Recently this result has been confirmed
by VLA observations of the CO J=1−0 transition (Greve et al. 2004). After many unsuccessful attempts CO has now been detected also in 4C 41.17. Similar to what was
found for 4C 60.07, De Breuck et al. (2004) report that the CO is distributed in two massive (> 5 × 1011 M ) components. For both galaxies the CO components have been
indicated on the 2-D spectra in Figures 4 and 7.
The presence of two separate molecular reservoirs near both galaxies suggests that
mergers play a fundamental role in the triggering of AGN and starburst activity in
HzRGs. Interestingly, similar velocity centroids are observed for the molecular and
emission line gas of the filaments of 4C 41.17 and 4C 60.07. This may indicate a connection between the filaments and the gas, and is consistent with a superwind scenario.
4.5 Metalicity
The discovery of both extended oxygen and CO lines throughout the emission line
nebulae prompts an important question: Is the extended gas enriched or is it pristine
primordial material? This is particularly important in considering whether enriched
material from the nuclear regions can escape from the deep potential wells via large
scale outflows and enrich the IGM.
For distant H II regions two methods are commonly used to determine the metalicity. One is based on the abundance of nitrogen, and the other on the abundance of
oxygen.
For HzRGs N V is often used as a reliable tracer of the abundances, because it is
rather insensitive to shock ionization, density fluctuations, ionization parameter, and
has a quadratic dependence on the metalicity (Villar-Martı́n et al. 1999). Vernet et al.
(2001) presented a metalicity sequence for HzRGs that is similar to the one for quasars
(Hamann & Ferland 1993). 4C 41.17 was included in their study and a metalicity of Z
∼ 1.3 Z was inferred for the central region (aperture of 2 00 × 1 00 Dey et al. 1997).
136
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
The oxygen based indicator Pagel et al. (R23 ; 1979) does not place strong constraints
for sources hosting luminous AGN. Still, the extent and luminosity of the oxygen lines
confirm not only that there must have been at least one significant generation of massive stars, but also that this must have happened early in the formation of 4C 41.17 for
the oxygen to have spread throughout the halo.
4.6 Mass estimates
One of the outstanding questions in galaxy formation is how the bulk of galaxy mass
assembles. In scenarios of monolithic collapse this is expected to be more or less instantaneous, whereas in models of hierarchical structure formation the mass is envisaged
to grow gradually over long time scales. Determining accurate masses for distant objects can help differentiate between the two scenarios.
4.6.1 Luminosity based masses
Typically, the K-band magnitudes of HzRGs suggest that they are already massive systems (e.g., De Breuck et al. 2002; Rocca-Volmerange et al. 2004). For example, Graham
et al. (1994) inferred for 4C 41.17 a stellar mass of order 20 L ∗ .
Assuming an absence of absorption by dust, a mass limit for the nebula can be obtained also from photoionization modelling of the total Lyα flux. In Paper I we thus
1/2
inferred gas masses of the order 0.3 − 1.3 × 1010 × f v,−5 M , where f v,−5 is the volume
filling factor of the extended Lyα emitting gas relative to f v = 10−5 . For f v ∼ 1 this
amount is substantial and comparable to that of fully formed, massive elliptical galaxies in the local Universe. Similarly, assuming a filling factor of f v = 10−5 , Villar-Martı́n
et al. (2003) inferred masses 109−10 M with densities in the range ∼ 17–150 cm−3 for 9
extended emisison line halos associated with radio galaxies at z ∼ 2.5.
4.6.2 Dynamical mass estimates
van Ojik et al. (1997) and Villar-Martı́n et al. (2003) have presented dynamical mass estimates for the more quiescent parts of the emission line regions in their studies. There
are two methods of estimating the dynamical masses. First, one can assume that the
halos consist of gas that has settled in rotating disks. Secondly, the halos can be envisaged as consisting of virialized clumps, which have velocity dispersions that balance
the gravitational forces.
Rotating disks
In the case of rotating disks, the mass can be estimated by measuring the velocity
2
rot
shear across the halo and using: Mdyn
= GRV
with R the radius of the disk, V half the
sin2 i
amplitude of the rotation curve, and i the inclination of the disk with respect to the
plane of sky. Villar-Martı́n et al. (2003) found evidence for rotation in 5 out of 9 halo
rot
velocity fields and inferred masses of order Mdyn
× sin2 i ∼ 0.3 − 3 × 1012 M .
For 4C 41.17 the measured relative velocity distributions beyond the radio lobes
show a velocity shear of ∼ 300 km s−1 (+100 km s−1 to −200 km s−1 ) at PA = 76 ◦ , over a
rot
distance of R = 20 00 /2 = 77 kpc. From this we infer a dynamical mass of about Mdyn
×
Section 4. Discussion
137
sin2 i ∼ 4 × 1011 M . The velocity profiles at position angles PA = 19 ◦ , and 21 ◦ show no
signs of a shear, consistent with their approximate alignment along an axis of rotation.
Similarly, for 4C 60.07 a velocity shear of ∼ 400 km s−1 (+200 km s−1 to −200 km s−1 ) at
PA = 81 ◦ over a distance of R= 7 00 /2 = 25 kpc yields an inferred dynamical mass of
rot
Mdyn
× sin2 i ∼ 3 × 1011 M .
For comparison, CO observations by De Breuck et al. (2004) and Papadopoulos et al.
CO
CO
(2000) yield dynamical mass estimates Mdyn
× sin2 i > 5 × 1011 M and Mdyn
× sin2 i >
8 × 1011 M for 4C 41.17 and 4C 60.07 respectively.
Pressure supported clouds
In contrast to the extended emission line halos of 4C 41.17 and 4C 60.07, the velocity
field for B2 0902+34 does not show evidence for rotation. We therefore also estimate
masses for the halos if they would be supported against gravitation by velocity disperions of the cloudlets in the halo. For this scenario the dynamical mass is given by:
vir
Mdyn
= 5RVR2 /G, with VR the radial velocity dispersion of the clouds (Carroll & Ostlie
vir
1999). With this dynamical masses for the outer regions of Mdyn
= 5 − 10 × 1012 M are
inferred. For the inner regions, where signal-to-noise is better, we find lower masses of
vir
Mdyn
∼ 2 × 1012 M .
4.7 Cosmological Implications
Absorbing gas associated with low velocity outflows has now been observed in several
HzRGs (van Ojik et al. 1997; Jarvis et al. 2003). Almost all of these show asymmetric
Lyα profiles suggesting the presence of blue-shifted absorbing gas which appears to
be spatially extended over the entire emission line region (see e.g., Dey 1999). Spectroscopic evidence for outflows at high redshifts exists also for other, presumably less
< L∗ ), and less metal enriched (Z ∼ 0.3 Z ) galaxies (Pettini et al. 2001; Dawmassive (∼
son et al. 2002; Shapley et al. 2003).
There is an ongoing debate whether massive (> L∗ ) galaxies or less massive (< L∗ )
galaxies enrich the ICM. Martin (1999) and Heckman et al. (2000) found that outflow
speeds are largely independent of galaxy mass. This would imply that smaller galaxies are more efficient at ejecting enriched material to large radii. Specifically, Heckman
(2002) claim that for massive galaxies the metals will not escape the deep potential
wells. However, recent modelling suggests that this needs to be true only for the most
massive halos (vesc > 600 km s−1 ) before discrepancies with the mass/metalicity relation become significant, and that the bulk of the metals in clusters are produced by L ∗
and brighter galaxies (e.g., Nagashima et al. 2004). Sensitive spectroscopic observations of ultra luminous infrared galaxies, now also find that more luminous galaxies
drive faster stellar winds (Martin 2004), and that the terminal outflow velocities always
approach the galactic escape velocity.
For all three HzRGs we have found features that can be interpreted as signs of
outflows. The extended optical filament of 4C 60.07 is reminiscent of an outflow in the
plane of sky. The blue-shifted carbon-rich component of B2 0902+34 could be the result
of an outflow of enriched material with an outflow velocity of up to 1000 km s−1 . The
near-infrared [OII] spectroscopy (Figs. 4, 5, 13), and earlier optical Lyα spectroscopy
138
Chapter 7. Metal enriched gaseous halos around distant radio galaxies
of 4C 41.17 (Fig. 4; Dey et al. 1997), show that both emission lines exhibit large blueshifted velocities ∼ 600 − 900 km s−1 (in projection) along the radio axis. In particular, the gas is very disturbed along the south-west filament, with Lyα velocity widths
ranging up to ∼ 900 − 1600 km s−1 . Beyond the radio hotspot, the velocity and velocity
widths decrease abruptly. The kinematics, the radial filamentary structure, and chemical enrichment of the gas all indicate a process of entrainment of material away from
the central regions by the radio jet. In this scenario, the optical filament represents the
shocked radiative cocoon of the radio source.
The multiple component, asymmetric, and twisted radio structure of 4C 41.17 suggests that the central SMBH has experienced multiple periods of radio source activity
and precession, presumably triggered by the interaction with one of the many clumpy
components which make up the complex galaxy structure (van Breugel et al. 1999;
Steinbring et al. 2002). The precessing radio source excavates the central region of the
galaxy, exposing surrounding material to a strong radiation field and mass outflow
from the active nucleus.
For a galaxy mass of 1 × 1012 M , which seems typical for HzRGs, the escape velocity is approximately vesc ∼ 400 km s−1 . The velocity gradients found from the spectroscopic observations are a factor 1.5–2.5 larger. While it may still be true that purely
starburst powered super galactic winds may not be sufficiently energetic for significant amounts of enriched nuclear material to escape from the galactic potentials, the
additional driving forces of the central AGN and the path cleared by the radio source
seem able to overcome this easily. Therefore, entrainment of nuclear material seems a
viable scheme of enriching the ICM.
The time scales for star formation and radio source activity in 4C 41.17 are very similar (Chambers et al. 1990; Bicknell et al. 2000) and comparable to that for transporting
the [O II] gas along the filament out to the vicinity of the south-west hotspot (∼ 7 × 107
yrs at ∼ 900 km s−1 ). It suggests an overall picture where the growth of the galaxy
through merging, the triggering of SMBH and starburst activity, and the enrichment of
its environment through outflows are all closely coupled processes.
5 Conclusion
One of our most important new results is the discovery of very extended [O II] and
[O III] emission associated with the gaseous halos of HzRGs. In particular, for 4C 41.17
[O II] emission has been detected as far as ∼ 60 kpc from the nucleus, providing evidence that the Lyα nebula, at least in this direction, is not only the result of scattering
but must be locally ionized. Furthermore, the presence of the oxygen emission lines
shows that the extended halo gas is not chemically pristine gas falling into the galaxy,
but is instead derived from regions of active star formation, and possibly transported
outward from regions located closer to the nucleus.
The evidence presented here could help cast observational light on the amazingly
good correlation found in galaxies between the stellar velocity dispersion and the black
hole mass (Gebhardt et al. 2000; Ferrarese & Merritt 2000). Silk & Rees (1998) and
Sazonov et al. (2004) have speculated that this results from outflows which are driven
by the radiation pressure provided by the black hole. These serve to limit the ratio of
Section 5. Conclusion
139
the black hole mass to bulge mass by ejecting the excess gas when such outflows can
overcome the galactic potential. In the case of 4C 41.17 we have good evidence that
the radio lobes are driving this outflow, assisted by radiation pressure and starburst
winds. Precession of the axis of ejection is likely to gradually clear out the gas in the
halo over a time period comparable with the dynamical timescale of the halo. Thus, for
4C 41.17 we may be observing the moment where galaxy collapse gives way to mass
ejection. These processes may help shape the high-mass end of the galaxy luminosity
function.
Acknowledgements
It is a pleasure to thank Carlos De Breuck and Andrew Zirm for stimulating discussions. We thank all staff at the W.M. Keck Observatory for their excellent support.
The authors wish to recognize and acknowledge the very significant cultural role and
reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most grateful to have the opportunity to conduct observations
from this mountain. M.R. thanks Mario Livio for generous hospitality at the Space Telescope Science Institute. The work by M.R., W.v.B., W.d.V., and S.A.S. was performed
under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory
under contract No. W-7405-Eng-48. The work of D.S. was carried out at Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. M.D.
acknowledges the support of the ANU and the Australian Research Council (ARC)
for his ARC Australian Federation Fellowship, and also under the ARC Discovery
project DP0208445. This work was supported by the European Community Research
and Training Network “The Physics of the Intergalactic Medium”.
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B2 0902+34
Object
Line
4C 41.17
4C 60.07
Lyα
C IV(doublet)
He II
[O II]
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FWHM
Å
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73.9 ± 2.7
59.9 ± 4.7
82: ± 30
45: ± 20
54.6 ± 15
50.8 ± 7.8
58.4 ± 3.0
FWHM
km s−1
1233 ± 13
1003 ± 65
718 ± 37
778 ± 62
2425 ± 12
1475 ± 132
2820 ± 104
1963 ± 156
1361: ± 500
754: ± 335
701 ± 271
640 ± 98
728 ± 37
Observed flux
×10−17 erg s−1 cm−2
49.4 ± 0.5
11.2 ± 0.4
8.9 ± 0.3
71.2 ± 7.5
41.1 ± 0.2
5.0 ± 0.3
9.5 ± 0.3
0.2 ± 0.8
267: ± 100
75: ± 40
34.8 ± 11.6
129.4 ± 20.5
484.7 ± 22.6
Table 3 — Summary of Emission-Line Measurements of the galaxies in this program
EWobs
Å
431 ± 12
109 ± 5
127 ± 7
310 ± 370
4976 ± 497
597 ± 179
788 ± 68
14 ± 63
190: ± 80
400: ± 200
112 ± 54
259 ± 85
1031 ± 352
Section 5. Conclusion
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Nederlandse samenvatting
Introductie
D
doelstellingen die astronomen zichzelf stellen zijn ambitieus: het begijpen van
de vorming en de evolutie van het Heelal. In de sterrenkunde is het in het algemeen niet mogelijk om de objecten onder studie aan te raken. Ook is het meestal niet
mogelijk om hun omgeving te veranderen en te zien wat voor invloed dat heeft op
hun gedrag. Dit is in tegenstelling tot vele andere wetenschappen, en stelt astronomen
voor een extra uitdaging3 .
Gelukkig helpt de natuur ons een handje. Het feit dat de lichtsnelheid eindig is, impliceert dat licht van verre objecten lang onderweg is geweest wanneer wij het waarnemen4 . Dit licht bevat daarom informatie over de toestand van objecten zoals ze lang
geleden waren. Hoewel veel grootschalige processen in het Heelal slechts veranderen
over tijdschalen veel langer dan een mensenleven, is het hierdoor mogelijk hun evolutie te bestuderen. De truc is om objecten te vinden die intrinsiek hetzelfde zijn maar op
verschillende afstanden tot ons staan.
Het bepalen van grote afstanden in de sterrenkunde maakt gebruik van de uitdijing
van het heelal5 . Deze expansie werd voor het eerst ontdekt door Edwin Hubble. Verweg gelegen stelsels bewegen zich sneller van ons af dan dichterbij gelegen stelsels. Dit
leidt tot een fenomeen vergelijkbaar met het Doppler effect, waarvan de verandering
in toonhoogte van een passerende brommer of een sirene van een ambulance bekende
voorbeelden zijn (de toon is lager als het object zich van ons afbeweegt dan wanneer
het nadert). Licht wordt op de zelfde manier “uitgerekt” en korte golflengtes worden
langer (roder). In formule vorm: λwaargenomen = λuitgezonden × (1 + z). De grootheid z
noemt men de roodverschuiving. In de kosmologie wordt de roodverschuiving zoveel
gebruikt, dat men deze vaak zelf als maat van afstand gebruikt. Het licht van objecten
met een roodverschuiving z = 1 werd uitgezonden toen het Heelal de helft van zijn
huidige leeftijd had (volgens huidige schattingen leven wij nu 13.7 miljard jaar na de
oerknal). In dit proefschrift worden objecten bekeken met roodverschuivingen z =3–5.
Het Heelal was toen nog maar 1–2 miljard jaar oud.
E
3
Hoewel in een vak als neurowetenschappen experimenteren mogelijk is kent het vergelijkbare problemen: ontleed een brein om de werking te bestuderen, en het houdt op te werken.
4
Reizend met een snelheid van een snelle sportwagen, zeg 200 km/h, duurt het 85 jaar om de Zon te
bereiken, 23 miljoen jaar om de dichstbijzijnde ster Proxima Centauri te bereiken, en langer dan 100 miljard jaar om ons eigen Melkwegstelsel te verlaten in het vlak van rotatie. Daarom drukken astronomen
afstanden uit in zogeheten parsecs (pc; en vaak zelfs kilo pc en Mega pc). De parsec is gedefinieerd als
de afstand waarop de straal van de baan van de Aarde om de Zon een boogseconde beslaat. De auto
uit ons voorbeeld zou er 17.6 miljoen jaar over doen om een parsec af te leggen. Licht reist veel sneller
(ongeveer 300.000 km/s), maar heeft er nog altijd 3.26 jaar voor nodig.
5
Voor kleine tot middellange afstanden wordt een verscheidenheid aan methodes gebruikt, die onafhankelijke schattingen voor afstanden geven. De grote afstanden in de sterrenkunde zijn gekalibreerd
aan deze zogeheten afstandsladder
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Nederlandse samenvatting
144
Ly α
CIV HeII
CIII]
Figuur 14 — Optisch 1-D spectrum van het radiostelsel 4C 60.07, besproken in Hoofdstuk 7. De prominente emissielijnen Lyα, C IV, He II, en C III] zijn aangeven. Gebaseerd op de He II λ1640 lijn wordt de
roodverschuiving bepaald op z = 3.7887 ± 0.0007. De inzet is een vergroting van het UV gebied van het
spectrum, getransformeerd naar de rust golflengtes zoals het spectrum in het stelsel werd uitgezonden.
Het bepalen van roodverschuivingen gaat het eenvoudigste door het meten van de
golflengte van verschoven emissielijnen (licht) met een bekende rustgolflengte. Emissielijnen zijn geassocieerd met specifieke atomen. Elektronen bevinden zich in banen om
de kern van het atoom. Als een atoom geëxciteerd wordt (bijvoorbeeld door botsingen
of bestraling) kan een elektron losspringen, of tijdelijk verschuiven naar een grotere
baan met meer energie. Als na verloop van tijd het atoom weer terugvalt naar zijn
grondtoestand komt de opgenomen energie weer vrij. Doordat, op grond van de
quantumfysica, slechts bepaalde banen zijn toegestaan, komen er bij het verspringen
van de ene naar de andere baan discrete hoeveelheden energie vrij met elk hun eigen
golflengte. Als nu de rustgolflengte bekend is, kan men uit het geobserveerde spectrum (vgl. een prisma dat zonlicht omzet in een regenboog) van een stelsel de roodverschuiving bepalen. Figuur 14 is een weergave van zo’n spectrum voor een (radio)
sterrenstelsel. Dit figuur laat een relatief zwak continuum zien met daarbovenop de
emissielijnen van verschillende elementen. Het continuum komt grotendeels van sterren en de emissielijnen van gas dat door straling van de sterren (en door de straling
geassocieerd met een zwart gat) geëxciteerd wordt. Als een spectrum maar één lijn bevat is identificatie daarvan lastig. Gelukkig zijn er in dit spectrum vier lijnen duidelijk
waarneembaar, en omdat ze alle vier dezelfde roodverschuiving moeten geven is er
maar één oplossing mogelijk: de roodverschuiving is z = 3.8. Het licht is dus 11.5
miljard jaar onderweg geweest voordat het door onze telescoop werd opgevangen.
145
Zoals Figuur 14 ook aangeeft, zenden atomen bij sommige golflengtes meer straling uit dan bij andere. De helderste lijn, Lyman alpha (Lyα), is geassocieerd met
geı̈oniseerd waterstof. Waterstof komt erg veel voor in het Heelal, ongeveer 73% van
massa van de elementen (25% is helium en de overige 2% is de rest, waaronder wij). Uit
deze mix van waterstof en helium onstaan de meeste sterren en melkwegstelsels. Door
kernfusie in sterren worden deze elementen langzaam omgezet in “afvalproducten”
zoals koolstof, zuurstof, ijzer etc. Zware sterren leven kort maar krachtig gedurende
een paar miljoen jaar. Tegen het einde van het leven van een zware ster, worden deze
afvalproducten in een grote supernova explosie de ruimte ingeslingerd. Relatief kleine
sterren dragen nauwelijks bij aan de verspreiding van de elementen, omdat ze veel
langzamer branden. Onze zon bijvoorbeeld is al 5 miljard jaar oud en zal nog zo’n 5
miljard jaar meegaan.
Als er genoeg afvalproducten bijelkaar komen, vormen ze grote stofwolken die tot
wel 1 miljoen zonsmassa’s6 kunnen bevatten. Omdat dit stof de binnengelegen gebieden afschermt tegen straling van buitenaf, kunnnen deze gebieden afkoelen. Daardoor kunnen ze zich samentrekken en uiteindelijk weer nieuwe sterren vormen. Vier
dingen zijn hier belangrijk. Ten eerste: stof is een product van stervorming. Ten
tweede: stof faciliteert verdere stervorming. Ten derde: de meeste stervormings gebieden zijn volledig aan het oog ontrokken door de gas en stofwolken waarin ze zich
bevinden. Het laatste belangrijke punt is dat nieuwe sterren het stof opwarmen, waardoor dit in het verre infrarood gaat stralen. Wij zullen de intensiteit van de straling
afkomstig van dit stof in Hoofdstuk 2, 3, 4, en 5 gebruiken als maat voor de snelheid
waarmee nieuwe sterren zich vormen.
Dit proefschrift
Sterrenstelsels worden geclassificeerd in twee verschillende hoofdsoorten. Er zijn majestueze spiraalstelsels die er schijfvorming uitzien en er zijn meer rugbybalachtige
elliptische stelsels. Verder zijn er nog tussenvormen en onregelmatige stelsels. Sterrenstelsels bestaan uit vaak wel 100 miljard sterren en verder grote hoeveelheden gas
en stof, en materiaal dat we niet kunnen zien (de zogeheten donkere materie).
Het doel van dit proefschrift is het bestuderen van de vorming en evolutie van
specifiek de grote en zware melkwegstelsels door gebruik te maken van waarnemingen van radiostelsels. We zullen verderop aangeven waarom juist radiostelsels hierover unieke informatie kunnen geven. Eerst volgt nu een kort overzicht van twee
populaire theoriën van de vorming van melkwegstelsels.
De algemeen aanvaarde theorie met betrekking tot het onstaan van het Heelal is
de zogeheten Oerknal theorie. Deze stelt dat het Heelal in het begin infinitesimaal
klein en heel heet was en hierna als gevolg van een soort explosie bezig is uit te dijen
en langzaam afkoelt. De gloed van deze explosie is nog steeds waarneembaar, de
kosmische achtergrond straling. De straling heeft een gladde verdeling, de afwijkingen
zijn van de orde 1:100.000. Dit geeft aan dat het Heelal in het begin en vlak na de
oerknal heel glad geweest moet zijn, vaak genoemd de oersoep. Als we echter naar het
lokale Heelal kijken, zien we dat dit bestaat uit sterren en sterrenstelsels met enorme
6
1 zonsmassa komt overeen met 2 × 1030 kg.
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Nederlandse samenvatting
hoeveelheden “lege” ruimte er tussenin. Het Heelal om ons heen is dus klonterig.
Een van de grote vragen in de sterrenkunde is: hoe en wanneer zijn sterrenstelsels
onstaan uit deze initiële afwijkingen in de dichtheidsverdeling en hoe zijn ze verder
geëvolueerd?
Er zijn twee populaire scenarios: het scenario waarin melkwegstels zich vormen
uit een enkele grote afkoelende gaswolk, en het scenario van hiërarchische structuurvorming. In dit laatste scenario vormen grote structuren pas laat, door het botsen en
samengaan van jongere kleinere objecten. In beide scenarios zijn pas geboren objecten
gasrijk. Het tweede scenario stelt dat botsingen een belangrijke manier zijn om het gas
zodanig samen te persen dat zwaartekracht de overhand krijgt en ze nog verder samen
doet trekken en er sterren onstaan.
Een recente ontdekking is dat waarschijnlijk (bijna) alle sterrenstelsels centrale zwarte gaten bevatten van een paar miljoen tot een miljard zonsmassa’s. Het blijkt dat de
massa’s van sterrenstelsels en hun centrale zwarte gaten met elkaar gecorreleerd zijn.
Dit is iets dat een theorie van de vorming van melkwegstelsels zou moeten kunnen
verklaren.
Hedendaagse theoriën van de vorming van melkwegstelsels doen goede voorspellingen voor het aantal stelsels dat een bepaalde massa heeft. Toch zijn er nog grote
verschillen tussen theorie en waarnemingen. Een van de discrepanties is dat men
op grond van deze theoriën een veel hoger aantal zware melkwegstels verwacht dan
wordt waargenomen. Hier is nog geen goede verklaring voor, behalve dat er waarschijnlijk een beperkend proces moet plaatsvinden dat relatief meer invloed heeft op
het groeiproces van zware stelsels. De preciese vorm hiervan is nog onbekend. Een
mogelijkheid is dat galactische winden, aangedreven door energie van stervorming en
zwarte gaten, het overtollig materiaal uit de stelsels blazen. Dit zou daarmee ook een
verklaring kunnen vormen voor het feit dat er ook “afvalstoffen” worden waargenomen
in de “lege” ruimte, waar slechts weinig of geen sterren worden waargenomen.
Radiostelsels
Verweg gelegen radiobronnen komen over het algemeen voor in de meest zware,
uitgestrekte en heldere stelsels. Er wordt aangenomen dat de radiostraling veroorzaakt wordt door de accretie van materiaal op ronddraaiende superzware (1 miljard
zonsmassa’s) zwarte gaten.
Omdat radiostelsels zo helder zijn, kunnen ze tot op grote afstand en dus tot vroeg
in het Heelal worden waargenomen. Het zijn daarom goede kosmologische bakens.
Bovendien is het feit dat ze zo uitgestrekt zijn erg handig om de melkwegstelsels zelf
te bestuderen. Omdat ze en massieve zwarte gaten bevatten en heel erg zwaar zijn,
zijn ze bij uitstek geschikt om de relatie tussen zwarte gaten en stervorming in het formatieproces van zware melkwegstelsels te bestuderen.
Een paar reeds bekende resultaten voor radiostelsels volgen hieronder:
Waarnemingen met de Hubble ruimtetelescoop hebben laten zien dat radiostelsels
met een hoge roodverschuiving uit vele kleine klontjes bestaan. Berekeningen tonen
aan, dat het waarschijnlijk is dat deze op tijdschalen van 100 miljoen jaar zullen bot-
147
sen en versmelten tot een groter object. Dit is consistent met het scenario van hiërarchische structuurvorming. Een andere voorspelling van het hiërarchisch model die
door radiostelsels bevestigd lijkt te worden, is dat zware radiostelsels gebruikt kunnen worden om de voorlopers van de grootste structuren in het Heelal, clusters van
sterrenstelsels, te vinden op hoge roodverschuiving.
In sommige verweggelegen radiostelsels is bewijs gevonden voor hoge stervormingssnelheden. Dit was voornamelijk gebaseerd op optische waarnemingen, en dus
onzeker door de mogelijke verduistering door stof. Recent is ook bewijs gevonden dat
dit niet slechts in kleine gebiedjes gebeurt maar door de hele stelsels. Het lijkt erop dat
deze stelsels op dat moment het grootste gedeelte van hun totale sterpopulatie aan het
vormen zijn. In hoofdstuk 2, 3, 4 en 5 hebben wij hier meer onderzoek naar gedaan.
Van verweggelegen radio stelsels is ook bekend dat ze zich bevinden in grote gasvormige halo’s. Het zou kunnen dat ze zich hieruit aan het vormen zijn. Een andere
mogelijke verklaring is dat dit gas is, dat naar buiten geblazen is vanuit de meer centrale gebieden. De precieze rol van de halo’s is nog niet bekend. Hoofdstuk 6 en 7
beschrijven de resultaten van onderzoek naar deze gaswolken.
Specifieke vragen die in dit proefschrift aan de orde komen zijn:
Wanneer vormen radio stelsels zich?
Wat is de rol van stof in dit vormingsproces?
Wat is de betekenis van de gashalo’s waarin ze zich bevinden?
Wat kunnen we van ze leren over de connectie tussen de groei van zwarte gaten en
stervorming?
Hoofdstuk 2
In Hoofdstuk 2 bestuderen we de stof emissie van een groot aantal verweggelegen
radiostelsels. Zoals we eerder hebben besproken wordt dit stof geproduceerd en verhit
door sterren, zodat er ver-infrarode straling ontstaat. De sterren zelf zijn door dit stof
grotendeels aan het oog ontrokken. Door nu de intensiteit van dit ver-infrarode licht
te meten op verschillende roodverschuivingen kan men hopen de geschiedenis van
de stervorming te bepalen. In dit hoofdstuk hebben wij een analyse gemaakt voor de
aldus afgeschatte stervorming in radiostelsels tijdens zeer verschillende periodes in
het Heelal. Het resultaat van deze analyse is dat de stervorming veel hoger was toen
het Heelal pas 1–2 miljard jaar oud was dan toen het 7 miljard jaar oud was. Een ander
resultaat is dat de stervormingsnelheid erg hoog is: er worden tot een paar duizend
zonsmassa’s per jaar aan nieuwe sterren gevormd. Dit is veel meer dan in ons eigen
Melkwegstelsel, dat slechts een stervormingsnelheid van ongeveer 3 zonsmassa’s per
jaar kent. Het belang van deze vinding is dat stelsels in staat lijken in 100 miljoen jaar
ongeveer 100 miljard sterren te vormen. Dit is vergelijkbaar met de massa van een
volledig melkwegstelsel. Het lijkt er dus op dat radiostelsels zich redelijk snel in een
groot kosmisch vuurwerk vormen, waarna ze een wat rustiger leven leiden.
148
Nederlandse samenvatting
Hoofdstuk 3 & 4
In deze twee hoofdstukken bestuderen we stelsels waarin de radiobron pas net is
gaan werken. Het blijkt dat deze stelsels nog meer verduisterd zijn door stof dan de
stelsels besproken in Hoofdstuk 2, en vergelijkbare of zelfs hogere stervormingssnelheden hebben. Hieruit concluderen wij dat het ontstaan van radiojets en een erg hoge
snelheid van stervorming waarschijnlijk aan elkaar gerelateerd zijn.
Hoofdstuk 5
Een complicatie bij de interpretatie van heldere ver-infrarode straling is dat stof niet
alleen door sterren maar ook door straling geassocieerd met zwarte gaten verhit kan
worden. In dit hoofdstuk hebben wij daarom de bijdragen van deze twee componenten gemodelleerd en vergeleken met waarnemingen. Uit deze vergelijking blijkt dat
de emissie geassocieerd met zwarte gaten tot wel 60% van de ver-infrarode lichtkracht
kan veroorzaken. Deze fractie neemt niet toe met toenemende lichtsterkte. Wel lijkt het
zo te zijn dat krachtigere bronnen hogere stof temperaturen hebben. Omdat een object
efficiënter straalt als het heter wordt, en er een niet-stervormingsgerelateerde component is, kunnen de stervormingssnelheden voor heel heldere objecten in het vroege
Heelal met een factor 2–3 naar beneden bijgesteld worden.
Hoofdstuk 6 & 7
In deze laatste twee hoofdstukken bestuderen wij de gaswolken waarin verre radiostelsels zich bevinden. Stelsels onstaan uit afkoelende gaswolken. Deze gaswolken
kunnen bijvoorbeeld stralen omdat ze gravitationele energie moeten kwijtraken voordat ze verder kunnen samentrekken, of omdat ze verlicht worden door centrale stervormingsgebieden en straling van actieve zwarte gaten. Omdat de gaswolken voornamelijk uit waterstof bestaan is de dominante straling de eerder beschreven Lyman
alpha emissie. Het is mogelijk om filters te maken die precies het golflengtegebiedje
van de roodverschoven Lyα omvatten. Deze filters hebben daardoor weinig last van
andere storende straling en zijn dus heel gevoelig. Wij hebben zo’n filter geı̈nstalleerd
op een van de grootste optische telescopen ter wereld (de Keck Telescoop met een 10m spiegel), en hiermee drie radiostelsels, waaronder die van Figuur 14, geobserveerd.
Een van deze waarnemingen is de meest gevoelige in zijn soort ooit gedaan.
De resulterende plaatjes zijn beschreven in Hoofdstuk 6 en laten enorme en spectaculaire gaswolken zien met een grootte tot 200 kpc. De gaswolken vertoonden nooit
eerder geziene langgerekte filamenten en het was ook niet eerder bekend dat ze zo
uitgebreid waren. Een verklaring voor de uitgebreidheid van deze wolken is dat dit
gas is dat vanuit grote afstand naar binnen valt en waaruit deze stelsels bezig zijn zich
te vormen. Een mogelijke verklaring voor de filamenten is dat ze een teken zijn van
grootschalige stromen die weggeblazen worden vanuit het centrum. Dit zou interessant zijn, omdat, zoals eerder gezegd, er een correlatie is tussen de massa van stelsels
en hun centrale zwarte gaten. Het is mogelijk dat door deze uitstromingen de groei van
het zwarte gat en stervorming op een zodanig zelfgereguleerde manier plaatsvinden
dat het tot de waargenomen correlatie leidt.
Hoofdstuk 7 gaat hierop verder, door de kinematica en de samenstelling van het gas
te bepalen. Er werd gevonden dat het gas erg hoge snelheden heeft op de locatie van
de radiobron. Bovendien werd gevonden dat het gas niet puur uit waterstof en helium
bestaat, maar dat er ook sterke sporen van zuurstof zijn met een vergelijkbaar snel-
149
heidsprofiel. Dit geeft aan dat het gas secundair materiaal is dat al eens onderdeel van
een ster is geweest. Een mogelijkheid is dat dit inderdaad verrijkt materiaal is dat door
de krachtige radiobron en sterrewinden vanuit de centrale stervormingsgedeelten van
het stelsel naar buiten wordt geblazen. Omdat je gas nodig hebt om sterren te vormen,
zouden zulke processen de groei van zware stelsels kunnen beperken. Deze waarnemingen zouden kunnen helpen om de theoretische en waargenomen massaverdelingen van stelsels met elkaar in overeenstemming te brengen.
Publications not included in this thesis
De Breuck, C., Downes, D., Neri, R., van Breugel, W., Reuland, M., Omont, A. & Ivison,
R., 2005, A&A, in press, “Dectection of Two Massive CO Systems in 4C 41.17 at z=3.8”
Dopita, M., Groves, B., Fischera, F., Sutherland, R., Tuffs, R., Popescu, C., Kewley, L.,
Reuland, M. & Leitherer, C., 2005, ApJ, in press, “Modelling the pan-spectral energy
distribution of starburst galaxies: I, The role of ISM pressure & the molecular cloud
dissipation timescale”
De Breuck, C., Bertoldi, F., Carilli, C., Omont, A., Venemans, B., Röttgering, H., Overzier,
R., Reuland, M., Miley, G., Ivison, R. & van Breugel, W., 2004, A&A, 424, 1, “A multiwavelength study of the proto-cluster surrounding the z=4.1 radio galaxy TN J13381942”
Scharf, C., Smail, I., Ivison, R., Bower, R., van Breugel, W. & Reuland, M., 2003, ApJ,
596, 105, “Extended X-Ray Emission around 4C 41.17 at z = 3.8”
Reuland, M., Röttgering, H. & van Breugel, W. 2003, New Astronomy Review, 47, 303,
“SCUBA observations of high redshift radio galaxies”
Stevens, J. A., Ivison, R. J., Dunlop, J. S., Smail, Ian R., Percival, W. J., Hughes, D. H.,
Röttgering, H. J. A., van Breugel, W. J. M. & Reuland, M., 2003, Nature, 425, 264, “The
formation of cluster elliptical galaxies as revealed by extensive star formation”
De Breuck, C., Neri, R., Morganti, R., Omont, A., Rocca-Volmerange, B., Stern, D.,
Reuland, M., van Breugel, W., Röttgering, H., Stanford, S. A. et al. , 2003, A&A, 401, 911,
“CO emission and associated H I absorption from a massive gas reservoir surrounding
the z = 3 radio galaxy B3 J2330+3927”
Dawson, S., Spinrad, H., Stern, D., Dey, A., van Breugel, W., de Vries, W. & Reuland,
M., 2002, ApJ, 570, 92, “A Galactic Wind at z=5.190”
151
Curriculum vitae
O
P 21 juni 1972 werd ik geboren in Groningen. Het grootste gedeelte van mijn
jeugd bracht ik door in Roden. Mijn middelbare school opleiding volgde ik aan
het Willem Lodewijk Gymnasium te Groningen alwaar ik in juni 1990 mijn diploma
behaalde in tien vakken.
In datzelfde jaar begon ik met mijn studie Sterrenkunde aan de Universiteit Leiden,
waar ik in 1991 mijn propedeutisch examen haalde. Onder leiding van dr. J.H.J. de
Bruijne, dr. R. Hoogerwerf en prof. dr. P.T. de Zeeuw heb ik tijdens de doctoraalfase
met behulp van computersimulaties onderzoek gedaan naar de kinematica en evolutie
van OB associaties. Dit onderzoek had als doel de specificaties van een toekomstige
astrometrische satelliet te helpen bepalen. In augustus 1999 behaalde ik mijn doctoraal
diploma. Tijdens mijn studie voerde ik van 1995 tot en met 1999 een eenmanszaak
gespecialiseerd in aanleg en beheer van computer netwerken.
Op 1 oktober 1999 begon ik met mijn promotieonderzoek dat beschreven is in dit
proefschrift. Dit onderzoek vond plaats in het kader van een samenwerkingsproject
tussen Leiden en Lawrence Livermore National Laboratories. Aan de Sterrewacht Leiden stond dit onderzoek onder leiding van prof.dr. G.K. Miley en dr. H.J.A. Röttgering.
Prof.dr. W.J.M. van Breugel had de leiding over het werk dat ik gedurende drie jaar
in de Verenigde Staten heb verricht. Daar was ik verbonden aan het Institute of Geophysics and Planetary Physics van het Lawrence Livermore National Laboratory te
Livermore, Californië en aan de University of California at Davis. Tijdens dit onderzoek ben ik vele malen op waarneemreis geweest naar telescopen op Hawaii (Keck,
JCMT), Spanje (IRAM) en Californië (Lick). Ook heb ik gebruik gemaakt van data
verkregen met de Hubble Space Telescope en het Chandra X-ray Observatory. Verder
legde ik werkbezoeken af aan de Landessternwarte Heidelberg (Duitsland), de Research School of Astronomy & Astrophysics van de Australian National University
(Australië) en bezocht ik een winterschool op La Palma (Spanje), een herfstschool in
Dwingloo en conferenties in Groningen, Leiden en Ile d’Oleron (Frankrijk).
Daarnaast heb ik tijdens mijn verblijf in Californië verscheidene Big Walls beklommen (El Capitan, Half Dome, Leaning Tower, Washington Column).
153
Nawoord / Acknowledgments
A
eerste wil ik iedereen bedanken die, bewust of onbewust, en elk op zijn/haar eigen
manier heeft bijgedragen aan de totstandkoming van dit proefschrift.
This project would not have been possible without the ideal scientific environments that
the IGPP/LLNL and the Sterrewacht have provided. In these times research depends heavily
on the smooth workings of computer networks. I would like to thank the respective computer
groups for ensuring that the systems were always operating at their finest.
Op een beslissend moment in mijn leven hebben Jos, Ronnie en hun begeleider mij als eerste
laten zien waar het in de wetenschap nu echt om gaat. Hiervoor ben ik hen erg dankbaar. Ik
had de uitdaging en spanning van de echte sterrenkunde die uiteindelijk geresulteerd heeft in
dit proefschrift niet willen missen.
During and outside working hours, the LLNL cosmology group: Wim, Jerry & Jenn, Brad,
Mark Lacy, and Steve provided much needed fun, discussion and relaxation. Dan Stern, and
Steve Dawson showed me the finer bits of Berkeley style astronomy.
De (ex-)bewoners van Vliet 11 en team “Bus 1+1” hebben er voor gezorgd dat ik me soms
ook bewust was van een wereld buiten de sterrenkunde. Speciale dank gaat natuurlijk uit naar
Lodewijk, die mij niet alleen de Vliet heeft binnengeloodst, maar ook in Livermore een prima
huisgenoot was. Lysbeth wil ik bedanken voor de gezellige discussies. We hebben veel van
elkaar geleerd.
The Sunrise Crew: Ed, Gary, Steve, Annie, Brent, Chad, Christina, Deke, Dan, Danny, Darlene, Dustin, Ed, Eric, Kendra, Kristine, Kyle, Jill, Jimmy, Jonathan, Luke, Little D., Matt, Mike,
Nick, Paul, Rob, Russ, Scott, Sunny, Tom, Tony. All of you made my stay in California unforgettable. Special thanks to those people who were crazy enough to go out on multi-day climbs
of ridiculously steeps walls with a guy from a land that has no mountains and where most of
the population lives below sea level. Paul, Luke, Brent and Tom, you have made my dreams
come true! I am sure many more great adventures are in store.
Op de Sterrewacht gaat mijn dank uit naar allen, maar in het bijzonder naar Andrew,
Roderik, Bram, Leonie, en Mariska. Voor mij was terugkomen in Nederland veel moeilijker
dan weggaan. Jullie hebben er voor gezorgd dat ik mij langzamerhand weer thuis voelde.
Dan zijn er nog de mensen die mij al vanaf kinds af aan kennen, en richting hebben gegeven
aan mijn leven. Mevrouw Roede, ik kan u niet genoeg bedanken voor de warme omgeving
waarin ik ben opgegroeid. Bart, in de loop der jaren hebben we teveel samen meegemaakt om
op te noemen. Ik hoop dat dit alleen maar meer wordt.
Natuurlijk ben ik ook mijn familie veel verschuldigd. Ascelijn, Marike, Merijn, en mijn
ouders hebben me altijd in alles gesteund en met veel liefde omringd. Dit proefschrift is voor
jullie allemaal.
Joke, jouw praktische insteek en eigen ervaring met het promoveren op afstand heeft me
geholpen dit project tot een succesvol einde te brengen. Zonder jou had ik geen halve marathon
gelopen, geen onbeklimbare bergen beklommen, en gewoon niet zoveel lol gehad. We maakten
van bergen hobbeltjes en van puinhopen een feest. Soms andersom, maar aan het einde telt dat
we er toch samen zijn geweest.
LS
155
Als je onzeker bent, schiet dan gewoon zo hard mogelijk. Als jij niet weet waar de bal
heengaat, weet de keeper het ook niet.
Johan Neeskens
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