when where and how are SMBHs Born

when where and how are SMBHs Born
T O APPEAR IN Nature Communications.
Preprint typeset using LATEX style emulateapj v. 5/2/11
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544
arXiv:1211.7082v1 [astro-ph.CO] 29 Nov 2012
This article documents our ongoing search for the elusive
“intermediate-mass” black holes. These would bridge the
gap between the approximately ten solar mass (M ) “stellarmass” black holes (the end-product of the life of a massive
star) and the “supermassive” black holes with masses of millions to billions of solar masses found at the centers of massive galaxies. The discovery of black holes with intermediate
mass is the key to understanding whether supermassive black
holes can grow from stellar-mass black holes, or whether a
more exotic process accelerated their growth only hundreds
of millions of years after the Big Bang. Here we focus on
searches for black holes with MBH ∼ 104 − 106 solar masses
that are found at galaxy centers. We will refer to black holes
in this mass range as “low-mass” black holes, since they are
at the low-mass end of supermassive black holes. We review
the searches for low-mass black holes to date and show tentative evidence, from the number of low-mass black holes that
are discovered today in small galaxies, that the progenitors
of supermassive black holes were formed as ten thousand to
one-hundred thousand solar mass black holes via the direct
collapse of gas.
Over the last decade we have come to understand that supermassive black holes, with masses of millions to billions of
times the mass of the Sun, are very common in the centers
of massive galaxies (Richstone et al. 1998). Black holes are
found in the centers of most massive galaxies at the present
time. We would like to understand when and how they formed
and grew.
We cannot yet watch the first supermassive black holes
form. They did so soon after the Big Bang, and light from
those distant events is beyond the reach of today’s telescopes.
However, we do have two very interesting limits on the formation of the first black holes. The first comes from observations
of the most distant known black holes: light is emitted by material falling into the deep gravitational potential of the black
hole. These monsters are so bright that they must be powered
by at least billion solar mass black holes. They had very little time to grow, as we see them only a few hundred million
years after the Big Bang (Fan et al. 2001). Whatever process
formed and grew the first massive black holes, it had to be
very efficient.
At the other extreme, we can study the lowest-mass black
holes in galaxy nuclei nearby to us, the left-over seeds that for
some reason never grew to be a billion suns. As we describe
below, conditions were best to make supermassive black hole
seeds soon after the Big Bang. Therefore, black holes found in
small galaxies today likely formed early and have not grown
significantly since. If we assume that black holes form in a
similar way in all galaxies, then the numbers and masses of
black holes in small galaxies today contains clues about the
formation of the first black holes (e.g., van Wassenhove et al.
2010). The sheer number of left-overs will indicate how commonly black hole seeds were formed, as well as inform future
gravitational wave experiments that expect to see a large number of paired low-mass black holes as they spiral together and
coalesce (Hughes 2002). Studying the energy output from
low-mass black holes could tell us whether growing black
holes at early times were important in shaping early star formation in the galaxies around them (e.g., Jeon et al. 2012).
Unfortunately, low-mass black holes are difficult to find.
Because of their low mass, they only have gravitational influence over stars in a very small volume at the galaxy center.
Therefore, we are often forced to wait until material falls into
the black hole. We detect the black hole indirectly via the
radiation energy that is released as matter falls in.
Here we document the last decade of searching for the elusive low-mass black hole population.
To understand the growth of the first supermassive black
holes, we first must determine how the black holes form to
begin with. Theoretically, there are two possible answers. Either black holes are created as the end-product of stellar evolution, a process that continues to produce stellar-mass black
holes today, or the black hole is made directly from the collapse of a gas cloud, which requires the high gas fractions
and low metallicities of the early universe. Once the black
hole is formed, it also must grow. There are likely many
growth paths, but a rapid mechanism is required to explain
the ∼ 109 M black holes that are observed only hundreds of
millions of years after the Big Bang (e.g., Fan et al. 2001). We
first discuss the two formation routes, and then the possible
growth mechanisms. Volonteri (2010) presents a very cogent
and recent review of the leading theories for the formation of
the first massive black holes. I will only briefly review the
subject for completeness, with an emphasis on the observable
consequences at the present day.
Stellar-mass black holes form when massive stars run out
of fuel at the end of their life. The first black holes may
have formed in the same way. The first stars were likely
very massive (e.g., Bromm & Yoshida 2011). In order for
stars to form, gas clouds need to contract; they are able to
cool and shrink by emitting light predominantly in specific
element transitions. Because there were no elements heavier
than He and Li in primordial gas, it was hard for clouds to
cool efficiently. As a result, proto-stars grew much larger before their gravitational attraction was strong enough to counteract the internal energy in the gas. In theory, the end-product
of these massive first stars will depend on the mass. Stars
with masses less than ∼ 100 M or more than ∼ 260 M will
make a black hole with a mass approaching that of the star
(e.g., Heger et al. 2003). For masses in between, it is thought
that pair-instability supernova, in which pair production in the
star center leads to a run-away stellar collapse, will leave no
remnant (e.g., Barkat et al. 1967). Of course, the details of
Big Bang
200 Myr
Track the growth of
black holes and
500 Myr
1 Gyr
Death of Massive
Direct Collapse
Few halos are seeded, but seeds
are ~104 solar masses
Black holes grow
via accretion and
3 Gyr
Most halos are seeded but
seeds are ~100 solar masses
Halos grow
via merging.
Some black holes are
ejected by gravitational
wave radiation.
6 Gyr
13.6 Gyr
~50% of ~109 solar mass
galaxies contain >105 solar
mass black holes
Today virtually all >1010 solar mass
galaxies contain supermassive
black holes
~90% of ~109 solar mass
galaxies contain ~104 solar
mass black holes
F IG . 1.— Schematic of the evolution of seed black holes assuming two different formation mechanisms (the death of the first generation of massive stars vs. the
direct collapse of gas into a black hole). Dark matter halos and the galaxies in them grow through merging. Black holes grow both via merging and by accreting
gas. One additional complication is that after merging, gravitational radiation “recoil” (see §3.1) may send the black hole out of the galaxy. At present, we can
distinguish between the two scenarios based on the fraction of small galaxies that contain massive black holes (we call this the “occupation fraction”).
early stellar evolution are very difficult to test observationally. There are many uncertain details, such as whether the
first stars formed in pairs, and how much mass they lose at
late stages of evolution. We will assume the first stars left
behind standard ∼ 100 M remnants.
Alternatively, conditions in the early universe may have allowed gas clouds to collapse directly into black holes (e.g.,
Haehnelt & Rees 1993). Direct collapse requires very low angular momentum gas that only existed in large quantities soon
after the Big Bang. In this scenario, only a very small fraction
of halos will manage to form a black hole, and only for a short
period of time soon after the Big Bang.
With these two formation paths in mind, the next question is
whether the black holes created via either path can grow into
the very luminous sources that are observed hundreds of millions of years after the Big Bang. With direct collapse models,
even in the halos with low angular momentum content and
low molecular hydrogen fraction (and thus inefficient cooling) the gas will likely settle into a disk, and require some sort
of instability to condense further (Lodato & Natarajan 2006).
Once sufficiently condensed, the central 104 − 105 M of material may very efficiently gain mass as a dense and round
“quasi-star”, (e.g., Begelman et al. 2006).
It is marginally possible for a stellar-mass seed to grow
into a billion solar mass black hole in hundreds of millions
of years, but only if the black hole manages to grow continuously at the maximal allowed rate. Above the so-called
“Eddington” limit, radiation pressure forces will exceed gravitational attraction and blow apart the accretion disk. In practice, it is difficult for black holes to grow continuously at their
Eddington limit, since the emission from accretion will heat
the gas around the black hole and slow down subsequent accretion (e.g., Milosavljević et al. 2009). One way to speed up
the growth of black holes created via stellar death is to merge
many smaller seeds into a more massive seed (e.g., Li et al.
2007). Dense clusters of stars contain many small seeds that
may sink to the center of the cluster and merge to form a more
massive seed with MBH ≈ 104 M that can then grow further
into a supermassive black hole (e.g., Miller & Davies 2012).
Similarly, stars may merge first, forming a supermassive star
and then create a more massive seed (e.g., Portegies Zwart
et al. 2004; Devecchi & Volonteri 2009).
3.1. Observational Consequences
These different formation scenarios are only interesting if
they predict differences in observations of the real universe.
Eventually, perhaps with the successor to the Hubble Space
Telescope, called the James Webb Space Telescope, we will
detect the earliest growing black hole seeds (e.g., Bromm &
Yoshida 2011). In the meantime, we can look for clues in how
black holes inhabit galaxies today. Just looking at supermassive black holes in massive galaxies provides few insights, because all memory of their humble beginnings has been erased
through the accretion of gas and smaller black holes. However, if we focus on the “left-over” seeds in small galaxies
(those with stellar masses Mgal < 1010 M ), the black holes
that never grew, we get a more direct view of the original seed
population (e.g., van Wassenhove et al. 2010).
Volonteri and collaborators construct models of dark matter halos merging and growing from the early universe to the
present day. They put seed black holes into the halos using different prescriptions depending on how the seeds were
formed (see Figure 1). Then, they watch the black holes
evolve along with the halos. There are still many uncertainties associated with these models. For instance, as black holes
merge, they emit gravitational radiation. In general, the gravitational radiation will have a preferential direction. When
the black holes finally merge, the remnant black hole will re-
Seed Black Holes
ceive a kick from the gravitational radiation that may, in extreme cases, send the black hole out of the galaxy completely,
called gravitational “recoil” (e.g., Merritt et al. 2004). Since
it is theoretically uncertain how effective gravitational radiation will be at ejecting black holes, there is additional uncertainty added to the models. Also, the models assume seeds
are formed either via direct collapse or via star death, when in
reality there is likely a mixture.
Given these uncertainties, we focus on the qualitative aspects of the models. They predict a higher fraction of lowmass galaxies to contain nuclear black holes if seeds are created via stellar deaths (see Volonteri et al. 2008 and the purple solid and green dashed lines, respectively, in Figure 2).
We are trying to measure the fraction of galaxies that contain low-mass black holes, particularly in host galaxies with
Mgal < 1010 M . As I will show, this work is still in progress.
Black holes revealed through
gas accretion
NGC 4395
Black holes revealed through
stellar kinematics
1 light
1 light
region gas
Gas Velocity
Larger distance
Broad emission lines show gas whizzing around the
central black hole.
4.1. Direct Detection with Stellar Dynamics
The most direct way to demonstrate that a supermassive
black hole exists at the center of a galaxy is to look for the
evidence of the gravity of the black hole from the motions of
gas or stars at the galactic nucleus. Just like planets going
around the sun, we can use the laws of gravity to translate the
average velocities of stars around the black hole into a mass.
At the center of our own Milky Way galaxy, researchers have
charted the motions of individual stars whipping around at the
galaxy center for over a decade. The star motions provide unambiguous evidence for a 4×106 solar mass black hole (Ghez
et al. 2008; Gillessen et al. 2009).
In other galaxies, we cannot study individual stars. We can,
however, still see the signature of the black hole in the average
star motions near the galaxy center. Stars move much faster
on average if there is a black hole at the center of the galaxy
then if there is not. The bigger the black hole, the faster the
average motions of the stars. Because the black hole comprises only a fraction of a percent of the total mass of the
galaxy, only the stars or gas very near the galaxy center can
feel the gravitational attraction of the black hole. To detect
these fast-moving stars requires either the high spatial resolution of the Hubble Space Telescope or adaptive optics from
the ground. In addition to stars, orbiting gas clouds can also
be used to weigh the black hole, using very similar principles (e.g., Barth et al. 2001; Herrnstein et al. 2005; Kuo et al.
It is only possible to detect the gravity of big black holes
that are in relatively nearby galaxies. As shown in the bottom
panel of Figure 4, the signal from the gravity of the black hole
is strongly concentrated towards the galactic center. When the
galaxy is further away, the motions of stars near the black hole
are blurred together with more distant stars that don’t feel the
black holes gravity, making its influence imperceptible. Similarly, as the black hole mass gets smaller, the high velocity
stars become more and more difficult to detect. To study lowmass black holes, we are generally forced to wait until matter
falls into the black hole and forms an accretion disk, which
we can detect.
4.2. Detecting accretion onto black holes
Occasionally, gas will make its way into the galaxy nucleus
and into the black hole. However, the gas must dissipate its
energy and angular momentum to fall into the deep gravitational potential of the black hole. Nature uses accretion disks
to funnel matter into black holes; accretion disks are many
hundreds to thousands of times smaller than the gas disks described in §4.1. Accretion disks are also hot, and radiate most
of their energy in the ultraviolet. While we cannot see the
peak of the radiation from the accretion disk, we can recognize the following signatures of an accreting black hole:
• Unresolved X-ray emission at the galaxy nucleus is a sign
of an accreting black hole. High-energy interactions between
photons and electrons form an X-ray corona above the accretion disk. Because the corona region is very compact, the light
signals propagate from one side of the corona to the other
rapidly, enabling variability on short timescales. Other processes can create X-rays in galaxies, but the X-rays are generally of lower energy, relatively fainter, and do not tend to vary
on the rapid timescales seen in accreting black holes.
• Sometimes accretion onto central black holes is accompanied by jets of accelerated particles that emit radio waves. Jet
emission often accompanies accretion disks, although the exact mechanisms responsible for launching the jets in accreting
supermassive black holes are not fully understood.
• Outside the accretion disk is gas that orbits the black hole
and emits spectral line transitions: for instance hydrogen
atoms emit optical light when electrons fall into the second
energy level. Intrinsically, light from electron transitions are
emitted at a specific frequency. However, because the gas is
moving towards and away from us, the frequency we observe
is shifted to the blue or the red via the Doppler shift. The
faster the gas is moving, the wider is the range of velocities
that we observe in line emission. The fast-moving gas that
orbits close to the black hole but outside the accretion disk is
called the “broad-line” region because of the high observed
• The gas in the galaxy on larger scales is also illuminated
by emission from the accretion disk. Because the accretion
disk emits strongly in the ultraviolet and X-ray, the gas in the
galaxy is excited to a wide range of temperatures. Gas that is
excited by accretion shows specific fingerprints in the ratios of
different atomic transitions. These emission line fingerprints
can be observed in the ultraviolet, optical, near-infrared and
mid-infrared wavelengths (e.g., Ho et al. 1997). For instance,
emission lines from quadrupley ionized Ne are typically only
excited in the vicinity of an accreting black hole (Satyapal
et al. 2007).
4.3. Using accretion to determine black hole mass
In cases for which we cannot determine a black hole mass
directly using the motions of stars or gas, we can get an ap-
proximate idea of the mass by observing the motions of gas
clouds in the broad-line region. We use the gas clouds in a
very similar way to the stars: the faster the gas moves on average, the wider the range of velocities we observe in the gas.
The wider the range of observed velocities, the more massive
the black hole. However, we need to know not only how fast
the gas clouds are moving, but also how far away they are
from the central black hole. Thinking about the planets in the
solar system as an analogy, we know that Pluto moves much
more slowly around the Sun than the Earth just because it is
at a much larger distance from the Sun. In the case of the gas
clouds, it is actually quite challenging to determine their distances from the black hole. In some cases the time delay in
variable emission from the accretion disk itself, and from the
broad-line region further away, provides a size scale for the
emission region, because the distance is just the delay time
times the speed of light. Using this technique, the mass of the
black hole in NGC 4395 is found to be MBH ≈ 105 M (Peterson et al. 2005; Edri et al. 2012). Measuring these time delays is very time consuming. Usually, we do not measure the
broad-line region size directly. Instead, we use a correlation
between the luminosity of the black hole and the broad-line
region size to estimate a radius for the broad-line region gas.
Astrophysical supermassive black holes were first discovered as “QSOs” – quasi-stellar objects with very high intrinsic
luminosities and very small sizes (e.g., Schmidt 1963). By the
late 1970s there was compelling evidence that QSOs are powered by accretion onto a supermassive black hole. They were
called “active galactic nuclei” (AGN) because they were shining via the energy released as material falls into (or is accreted
by) the central supermassive black hole (Lynden-Bell 1969).
The existence of real supermassive black holes, with masses
of hundreds of millions of times the mass of the sun became
commonly accepted, but it was far less clear whether these
“monsters” represented a rare and long-lived phenomenon, or
whether all galaxies contained supermassive black holes with
short-lived bright episodes.
Twenty years later we finally learned that supermassive
black holes are common. In fact, we believe that most massive galaxies contain a central supermassive black hole. The
evidence came from both stellar dynamics and accretion (see
text box). A survey by Ho, Filippenko, & Sargent (Ho et al.
1997) searched the centers of nearby, “normal” galaxies for
subtle evidence that trace amounts of gas was falling into a
central black hole. Amazingly enough, most (∼ 70%) massive
galaxies showed clear signs of accretion onto a supermassive
black hole (see review in Ho 2008). At the same time stellar dynamical work was providing increasing evidence that
every bulge-dominated galaxy harbors a supermassive black
hole (e.g., Richstone et al. 1998). It became clear that black
holes were preferentially associated with galaxy bulges1 . Furthermore, the ratio of black hole to bulge mass was apparently
constant to within a factor of two to three (Tremaine et al.
Unfortunately, understanding the black hole population becomes increasingly challenging as one considers lower and
lower-mass galaxies. Low-mass galaxies typically contain
1 Bulges are ellipsoidal in shape and comprised of mostly old stars that
move on random orbits through the galaxy. In contrast, disks are flat components of galaxies, where stars all orbit the galaxy on coplanar circular paths.
Disks contain gas and ongoing star formation. If a bulge component contains
no disk, we call it an elliptical galaxy.
more cold gas, more dust, and higher levels of ongoing star
formation. The dust obscures emission from accretion, while
the star formation masks it. Furthermore, if the correlation
between BH mass and bulge mass applies, the BHs in smaller
galaxies are less massive, which makes their emission weaker.
Lower-mass black holes also exert a smaller gravitational
force, so that it becomes more and more challenging to detect stars moving under the influence of the black hole.
5.1. Dynamical Evidence for Black Holes in Low-mass
For the handful of bulgeless galaxies nearest to us, it is
possible to search for the gravitational signature of a central black hole. In stark contrast to bulge-dominated galaxies, these nearby bulgeless galaxies show no evidence for a
central massive black hole, with an upper limit of 1500 M
for the galaxy M33 (Gebhardt et al. 2001) and of 104 M for
NGC 205 (Valluri et al. 2005). Lora et al. (2009) and Jardel &
Gebhardt (2012) find that the black hole masses of two very
low-mass dwarf galaxies in our local neighborhood cannot
be larger than . 104 M . While massive, bulge-dominated
galaxies contain black holes (an “occupation fraction” close to
unity), clearly the occupation fraction drops in galaxies without bulges. But, do 50% of dwarf galaxies contain black holes
or only 1%? And how does that fraction change with the mass
of the galaxy?
Apart from M33 and NGC 205, there are very few galaxies near enough to place interesting limits on the presence
or absence of a black hole based on the motions of stars or
gas at the galaxy center (see text box above). Barth et al.
(2009) studied the nuclear kinematics of the bulgeless galaxy
NGC 3621. This galaxy also shows some evidence for accretion (see §3.3). They place a very conservative upper limit
of 3 × 106 M on the mass of a central massive black hole,
which would be improved by better measurements of the stellar ages in the star cluster surrounding the black hole. Seth
et al. (2010) used the motions of a gas disk in the center of
the small S0 galaxy NGC 404 to find a likely black hole mass
of ∼ 5 × 105 M . Neumayer & Walcher (2012) find upper
limits of ∼ 106 M for nine bulgeless spirals, confirming that
such galaxies contain low-mass black holes if they contain a
central black hole at all.
5.2. Bulgeless Galaxies with Active Nuclei
M33 taught us that not all low-mass galaxies contain central
supermassive black holes. The galaxy NGC 4395, a galaxy
very similar in mass and shape to M33, shows that some
galaxies without bulges do contain nuclear black holes (Filippenko & Sargent 1989). Like M33, NGC 4395 is small and
bulgeless. Unlike M33, NGC 4395 contains unambiguous evidence for a central massive black hole (see text box), including extremely rapid variability in the X-rays (Shih et al. 2003)
and a radio jet (Wrobel & Ho 2006). While we do not know
precisely, the black hole mass is likely 104 − 105 M (Filippenko & Ho 2003; Peterson et al. 2005; Edri et al. 2012).
NGC 4395 highlights the utility of using nuclear activity as
a fingerprint of low-mass black holes when their gravitational
signature is undetectable. In 2004, Aaron Barth reobserved
the forgotten active galaxy POX 52 (Kunth et al. 1987; Barth
et al. 2004), which has a near-identical optical spectrum to
NGC 4395. POX 52 also appears to contain a ∼ 105 M black
hole. NGC 4395 went from being an unexplained oddball to
the first example of a class of objects, with POX 52 being the
Seed Black Holes
Theory: Stellar Death
Theory: Direct Collapse
F IG . 2.— We show the expected fraction of galaxies with Mgal . 1010 M that contain black holes with MBH >
∼ 3 × 10 M , based on the models of Volonteri
et al. (2008), as presented in Volonteri (2010), for high efficiency massive seed formation (solid purple line) , as well as stellar deaths (greed dashed line). From
data in the literature, in large circles we show the fraction of galaxies containing black holes greater than 106 M (lower points) and greater than 3 × 105 M
(higher points) based on the paper by Desroches & Ho (2009) (blue) and Gallo et al. (2010) (red). See text for details. Although the uncertainties are very large,
we find tentative evidence in support of the efficient massive seed models (purple solid line).
second example. But are there more? We were inspired to
perform the first large systematic search for this new class of
“low-mass” accreting black holes.
In 2003, the Sloan Digital Sky Survey ( SDSS York et al.
2000) had just started to provide pictures and spectra of objects over one-quarter of the sky, exactly what we needed to
search for the rare and elusive low-mass black holes. We decided to find accreting black holes and use the motions of
gas very close to the black hole to trace the black hole mass
(see §4.2). We went through hundreds of thousands of galaxy
spectra to pick out the accreting black holes with fast-moving
gas (§4.2). We then picked out the ∼ 200 systems with masses
< 106 M (Greene & Ho 2004, 2007a). We chose this mass
because it is similar to the mass of the black hole at our Galaxy
center, and serves as the anecdotal low-mass cut-off of supermassive black holes. Dong et al. (2012) also searched through
the SDSS for low-mass black holes with similar criteria, increasing the total sample by ∼ 30%.
Subsequent searches of the SDSS have adopted different
approaches. While looking for galaxies with pristine, metalfree gas, Izotov & Thuan (2008) present evidence for accreting black holes in four vigorously star forming galaxies, again
based on the detection of fast-moving gas that is most likely
orbiting a black hole with MBH ∼ 104 − 106 M (see Fig. 4).
In contrast, a number of groups are now first selecting lowmass galaxies, and then searching for signatures of accretion
(e.g., Barth et al. 2009).
5.3. Multiwavelength Searches
Searches for low-mass black holes using the SDSS were
an important first step, and allowed us to comb through hundreds of thousands of galaxies. However, they are fundamentally limited in two ways. Firstly, in galaxy nuclei with ongoing star formation, dust obscuration and emission from star
formation hides the evidence of nuclear activity. Secondly,
the SDSS takes spectra of a biased sample of relatively bright
galaxies, which makes it very difficult to calculate a meaningful occupation fraction (Greene & Ho 2007b).
An obvious way to circumvent these biases, and complement the original optical searches, is to use other wavebands.
X-rays, for instance (see text box), are such high-energy photons that they can only be hidden by very large quantities of
gas. Radio and mid-infrared wavelengths are also relatively
unaffected by dust absorption. On the other hand, multiwavelength searches to date have been restricted to small samples.
The fraction of low-mass (Mgal < 1010 M ) galaxies with
X-ray emission coming from the nucleus has been studied
both as a function of stellar mass (Gallo et al. 2010; Miller
et al. 2012) and galaxy morphology (Desroches & Ho 2009;
Ghosh et al. 2008). The former studies focused on galaxies comprised of old stars, while the later focused on starforming galaxies. Less than 20% of non-starforming galaxies with Mgal < 1010 M have nuclear X-ray sources with
LX >
∼ 2.5 × 10 erg s , while in star forming galaxies of similar mass, ∼ 25% of galaxies contain X-ray nuclei above the
same luminosity. The difference in detection rate is likely due
to a lack of gas to consume in the red galaxies. In §6, I will
use these detection fractions to estimate the occupation frac-
Secular BH Fueling
NGC 3621;
Satyapal et al. 2007
Spitzer (+others)
F IG . 1.— Images of the three megamaser disk galaxies from our HST program that appear to contain disks on ∼ 500 pc scales. North is up and East to the left. For
each galaxy we show a F336W, F438W, F814W color composite, and then zoom in on the F160W image inside the boxed region. The arrows show the orientation
of the large-scale galaxy disk (G), the megamaser disk (M) and a jet if known (J; see also Table 1). Finally, we show the radial profile from ellipse, along with the
radial distribution in ellipticity and PA. The red dotted line notes the scale on which we measure the PA of the disk-like structure. Recall that the megamaser disks
are effectively edge-on in all cases, unlike the large-scale galaxies, and the arrow points towards the blue-shifted component of the disk.
Amy E. Reines1, Gregory R. Sivakoff1, Kelsey E. Johnson1,2 & Crystal L. Brogan2
mass uncertainty
©2011 Macmillan Publishers Limited. All rights reserved
The stellar masses of the galaxies (Fig. 1, middle) range from 4 × 108 M! to 1 × 1010 M! .
The nuclear properties of our sample are interesting. Only two objects (one of which is NGC
4395) are classified as type 1 AGNs. The other 24 are narrow-line objects with emission-line flux
ratios consistent with those of Seyfert 2 galaxies (Fig. 1, right). In general, the objects have
exceptionally weak lines. For example, the median luminsity of the [O iii] λ5007 line — a decent
indicator of the isotropic luminosity of an AGN (Heckman et al. 2005) — is just 6 × 1038 ergs s−1 .
Spectra and images of two objects from our sample are shown in Figure 2. Because of their
proximity, our IMBH candidates are ideal for follow-up study with Chandra.
Seth et al.
Figure 2: Two IMBH candidates from our survey, J0811+2328 (top) and J1009+2656 (bottom). Their
starlight-subtracted spectra in the Hβ (left) and Hα (center) regions are shown along with their SDSS
images, which have the same physical scale (15 kpc × 15 kpc). The galaxies are tiny: both have Mg = −17.2
and stellar masses of ∼ 2 × 109 M! .
Figure 2. SDSS colour cutouts of 15 AGN (shown in Figure 1 as green open circles) with potentially bulgeless host galaxies from visual
selection. Reading from top left, the images are sorted by ascending redshift, from z = 0.014 to z = 0.19, matching the order in Table 1.
NGC 4395
& Ho
6 6 | N AT U R E | VO L 4 7 0 | 3 F E B R U A RY 2 0 1 1
F IG . 2.— Same as Figure 1 above, but it is ambiguous whether these two galaxies contain nuclear disks.
Izatov & Thuan
Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, Virginia 22904, USA. 2National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22904,
Dong et al.
Figure 1 | Henize 2-10. Henize 2-10 is a blue compact dwarf galaxy hosting a
concentrated region of extreme star formation. Using Ha (ref. 8) and 24 mm
(ref. 11) fluxes from the literature, we estimate a star-formation rate12 of
1.9M[ yr21, assuming that all of the emission is from the starburst and that the
contribution from the active nucleus is negligible. We estimate that Henize 2-10
has a stellar mass of 3.7 3 109M[ from the integrated 2MASS Ks-band flux14,15.
Neutral hydrogen observations of Henize 2-10 indicate a solid-body rotation
curve typical of dwarf galaxies with a maximum projected rotational velocity of
39 km s21 relative to the systemic velocity of the galaxy7. These observations
also indicate a dynamical mass of about 1010M[ within 2.1 kiloparsecs (ref. 7).
The main optical body of the galaxy, shown here, is less than one kiloparsec
across. Henize 2-10 shows signs of having undergone an interaction, including
tidal-tail-like features in both its gaseous7 and stellar distributions (seen here).
In this three-colour HST image of the galaxy, we show ionized gas emission in
red (Ha) and the stellar continuum in green (,I-band, 0.8 mm) and blue (,Uband, 0.3 mm). These archival data were taken with Wide Field and Planetary
Camera 2 (Ha) and the Advanced Camera for Surveys (U- and I-band). The
white box indicates the region shown in Figs 2 and 3.
Milky Way
Vol. 706
Greene & Ho
& Ho
Galaxy Zoo : Bulgeless AGN Host Galaxies
are at least an order of magnitude less luminous than the central source
in Henize 2-10 (see Supplementary Information). In contrast, the radio
and hard X-ray luminosities of the central source in Henize 2-10, as well
as their ratio, are similar to known low-luminosity active galactic nuclei
powered by accretion onto a massive black hole21.
The central, compact, non-thermal radio source in Henize 2-10 is
also coincident with a local peak in Paa and Ha emission and appears to
be connected to a thin quasi-linear ionized structure between two bright
and extended regions of ionized gas. This morphology is tantalizingly
suggestive of outflow (Fig. 2). Although we cannot conclusively determine whether or not this linear structure is physically connected to the
brightest emitting regions with the data in hand, ground-based spectroscopic observations22 confirm a coherent velocity gradient along the
entire ionized gas structure seen in Fig. 2, consistent with outflow or
rotation. Moreover, a comparison between the central velocity of this
ionized gas structure and the systemic velocity of the galaxy—derived
Supermassive black holes are now thought to lie at the heart of every
giant galaxy with a spheroidal component, including our own Milky
Way1,2. The birth and growth of the first ‘seed’ black holes in the
earlier Universe, however, is observationally unconstrained3 and we
are only beginning to piece together a scenario for their subsequent
evolution4. Here we report that the nearby dwarf starburst galaxy
Henize 2-10 (refs 5 and 6) contains a compact radio source at the
dynamical centre of the galaxy that is spatially coincident with a
hard X-ray source. From these observations, we conclude that
Henize 2-10 harbours an actively accreting central black hole with
a mass of approximately one million solar masses. This nearby
dwarf galaxy, simultaneously hosting a massive black hole and an
extreme burst of star formation, is analogous in many ways to
galaxies in the infant Universe during the early stages of black-hole
growth and galaxy mass assembly. Our results confirm that nearby
star-forming dwarf galaxies can indeed form massive black holes,
and that by implication so can their primordial counterparts.
Moreover, the lack of a substantial spheroidal component in
Henize 2-10 indicates that supermassive black-hole growth may
precede the build-up of galaxy spheroids.
The starburst in Henize 2-10, a relatively nearby (9 megaparsecs,
,30 million light years) blue compact dwarf galaxy, has attracted
the attention of astronomers for decades6–10. Stars are forming in
Henize 2-10 at a prodigious rate8,11,12 that is ten times that of the
Large Magellanic Cloud13 (a satellite galaxy of the Milky Way), despite
the fact that both of these dwarf galaxies have similar stellar masses14–16
and neutral hydrogen gas masses7,17. Most of the star formation in
Henize 2-10 is concentrated in a large population of very massive
and dense ‘super-star clusters’, the youngest having ages of a few
million years and masses of one hundred thousand times the mass
of the Sun6. The main optical body of the galaxy has an extent less than
a kiloparsec (,3,000 light-years) in size and has a compact irregular
morphology typical of blue compact dwarfs (Fig. 1).
We observed Henize 2-10 at centimetre radio wavelengths with the
Very Large Array (VLA) and in the near-infrared with the Hubble Space
Telescope (HST) as part of a large-scale panchromatic study of nearby
dwarf starburst galaxies harbouring infant super-star clusters18–20. A
comparison between the VLA and HST observations drew our attention
to a compact (,24 pc 3 9 pc) central radio source located between two
bright regions of ionized gas (Fig. 2). These data exclude any association of this central radio source with a visible stellar cluster (Fig. 3;
see Supplementary Information for a discussion of the astrometry).
Furthermore, the radio emission from this source has a significant
non-thermal component (a < 20.4, Sn / na where Sn is the flux density
at frequency n) between 4.9 GHz and 8.5 GHz, as noted in previous
studies of the galaxy9. An archival observation of Henize 2-10 taken
with the Chandra X-ray Observatory reveals that a point source with
hard X-ray emission is also coincident (to within the position uncertainty) with the central non-thermal radio source10 (see Supplementary
Information). Typically, even powerful non-nuclear radio and X-ray
sources (for example, supernova remnants and active X-ray binaries)
An actively accreting massive black hole in the dwarf
starburst galaxy Henize 2-10
Black Hole Mass (M⦿)
Each cutout is marked at the top left with a scale bar representing 5!! .
tions (Simmons & Urry 2008). This additional uncertainty
particularly affects the faint, compact second host galaxy
components, but where a compact host component is detected we can nevertheless distinguish between bulge and
pseudobulge in all but one system (described in Section
3). However, when calculating the upper limit to a possible
bulge contribution
10 to the galaxy (such as in Figure 6), we include all the light from even those compact host components
firmly detected as pseudobulges. We therefore consider our
bulge limits conservative upper limits.
Are these AGN host galaxies really bulgeless?
The remaining sample of 13 host galaxies are all well fit by
a model consisting of a dominant disk and a nuclear point
11strong constraints on the maximum consource, providing
tribution of a small bulge component. In 3 cases, we do not
detect a second, compact host component. For the other systems, examination of residuals from fitting only the extended
disk + nuclear point-source components shows clear signs of
Figure 1. False-color HST images of the LBA sample showing the (rest-frame) UV in blue/purple and optical in yellow/red. The images measure 6!! ×6!! . The UV
a small
images were rebinned (2×2) to match the pixel scale of the optical images, and convolved with a Gaussian kernal with 0.!! 1 (FWHM). Although most objects
areextended component in the center; in all but one
case disk.
this additional component has a Sersic index consistent
highly compact in both the UV and optical, a small subset consists of a very bright unresolved component in the middle of an extended, low surface brightness
with a pseudobulge (n < 2 within the 1σ uncertainties). The
The images demonstrate a wide range of complex morphologies often suggestive of interactions and (post-)merging. See the text for details.
mean and median contributions of these pseudobulges to the
(A color version of this figure is available in the online journal.)
total host galaxy light are 3.6% and 3.3%, respectively.
By construction, the sample is unambiguously diskThe sole exception (J094112.93+610340.7) has a
one associated
source (J162511.78+504202.1)
with the DCOs in apFigure 3). Many
pears to
of a galaxy
a strong
(n =
of be
thea other
as faint
or n = 2.0 ± 0.7, meaning we cannot say whether
it 3).
is a classical bulge or a pseudobulge. Also, it is worth
2.65 ± larger
0.18) and
a companion,
tidal tails
(see that
in Figure
notingwethat the host galaxies with no detected pseudobspiral arms.
To Another
obtain the
best constraints on isthe
sizes simof the DCOs,
ulge are the highest-redshift sources in the sample; higherilar, but
a disk-dominated
= 0.6of
± 0.26)
the radial flux (n
the three brightest
resolution imaging could clarify both the status of this one
central component with a bulge-dominated companion. This
examples, using the ACS UV images as they have smaller
exception and the pseudo/bulgeless nature of those sources
system contains a broad-line AGN, and fitting the extended
and a sharper PSF compared to the optical images. The with
resultz is
> 0.06.
arms requires strong Fourier (asymmetric) modes. As the
in Figure
7. Each
panel isshows
the measured
count It
is therefore highly likely that most of these galaxies
of the
with a
PSF bulgeless. However, we consider that all of the light
are truly
physical interaction between it and the primary source, this
modeled using
the arms
these components may be bulge light and consider it a
is likely
a merger
or post-merger
and the
may in
We also
a rangeare
of two-dimensional
robust upper limit on the contribution of a classical bulge.
be tidally
Both galaxies
removed from
models having a FWHM of 0. 005–0. 125 and convolved
no bulge is detected, we assume the upper limit to be
our sample.
Figure 2. Stellar mass vs. the half-light radius measured in the UV (left panel)
with the model PSF. The results are shown using colored lines.
and optical (right panel) images. Solid squares mark the subset of DCOs in!
c 0000InRAS,
all three
data are most consistent with an object no
anticipation of results obtained in Section 5.2.
Stellar Mass (M⦿)
F IG . 3.— The relationship between galaxy bulge mass and black hole mass is linear for bulge-dominated galaxies, as shown by the solid red line in the upper
right from Häring & Rix (2004). To guide the eye, we have extrapolated this relationship down to lower black hole masses with the dotted red line. However, in
disk-dominated galaxies, particularly at low mass, there is no tight correlation between MBH and properties of the galaxy. We illustrate the wide range of galaxy
types hosting low-mass black holes, roughly placed in accordance with galaxy and black hole mass. However, note that only NGC 404 has a direct dynamical
black hole mass measurement. In all other cases, the black hole masses are very approximate, as illustrated by the error bar to the left.
Chandra Observations
Given the unbiased nature of our sample, it is very significant that 24/26 of the objects are type 2
AGNs with narrow emission-line spectra. In optical surveys of luminous galaxies, type 2 AGNs
outnumber type 1s by only a factor of 3 or 4 (e.g., Osterbrock & Shaw 1988). It’s possible, as
with luminous Seyfert 2s, that the broad-line regions of the AGNs in our sample are obscured by
a dense, parsec-scale torus (e.g., Antonucci 1993). But in this scenario, something would have to
be different about the typical geometry or covering factor of the torus in order to account for the
dominant type 2 fraction. On the other hand, it’s not clear exactly how the torus originates in
the first place, even in luminous AGNs. One possibility is that it might result from a wind driven
from the surface of the accretion disk by radiation pressure (Konigl & Kartje 1994). However, in
modeling such a wind, Elitzur & Shlosman (2006) find that AGNs with low bolometric luminosities
are not powerful enough to create a torus. In a similar vein, it has been shown (Nicastro 2000;
tion in galaxies with Mgal < 1010 M .
The X-ray luminosities probed here are very low. If a
105 M black hole has very little to accrete, it will only shine
very weakly, in this case with an X-ray luminosity as low as
LX . 1038 erg s−1 . However, stellar mass black holes can
sometimes shine with this luminosity as well, although they
are not very common; the best guess is that ∼ 10% of the
sources are actually powered by 10 M black holes while
the rest are powered ∼ 105 M black holes (e.g., Gallo et al.
2010). However, as we look at less and less luminous X-ray
sources, more and more of them will be powered by stellarmass black holes, until for LX . 1037 erg s−1 , nearly all the
sources will be stellar-mass black holes. Due to confusion
about the nature of the detected objects, we are reaching the
limit of what X-ray searches alone can tell us about the demographics of low-mass black holes. It is possible that including X-ray variability information will also help weed out the
stellar-mass black holes (Kamizasa et al. 2012).
The high-ionization [Ne V] line, detected in mid-infrared
spectroscopy, is a reliable indicator of AGN activity since
starlight likely cannot excite this transition (e.g., Satyapal
et al. 2007). Satyapal et al. (2008, 2009) focus on galaxies
with little to no bulge component. In galaxies with very small
bulges, they find a detection fraction similar to the X-ray studies (∼ 20%; see also Goulding et al. 2010) but in the galaxies with no bulge whatsoever, their detection fraction drops
nearly to zero (one galaxy out of 18 contains a [Ne V] detection). While the precipitous decline of detected bulgeless
galaxies provides evidence for a dramatic decline in the occupation fraction of bulgeless galaxies, it is worth noting that
the observations of the latter galaxies were not as sensitive.
Further progress requires observations at multiple wavelengths. For example, Reines et al. (2011) identified a likely
3.3. Clump Sizes
It is clear from simple visual inspection of the HST images
that the DCOs are marginally resolved at best (note the strong
105 − 106 M black hole in the center of the low-mass starforming galaxy Henize 2-10. Radio or X-ray emission alone
would have been unconvincing, since either could easily be
explained by processes relating to star formation. However,
the spatial coincidence of the radio and X-ray source, their
relative brightness, and their distance from clusters of forming stars make a compelling case for a low-mass black hole
in this galaxy. A larger sample of galaxies like Henize 210 (Overzier et al. 2009) also appear to have accreting black
holes in some cases. Again, it is the combination of X-ray and
radio detections that argues for black holes in these galaxies
(Jia et al. 2011; Alexandroff et al. 2012).
larger than ≈0. 075 (FWHM), corresponding to effective radii
no larger than $70–160 pc at z = 0.1–0.3).
In order to be consistent across our entire clump sample and
with our earlier definition of the clumps, we have measured the
physical sizes of the brightest clumps in each LBA by measuring
In addition to measuring the fraction of low-mass galaxies
that contain black holes, it is of interest to determine whether
black holes with lower mass emit a different spectrum than
more massive accreting black holes. For instance, one naively
expects that the accretion disk will get physically smaller and
thus hotter as the black hole mass decreases. In turn, gas in
the vicinity of the black hole will be heated by more energetic
photons. Predictions of the impact of low-mass black holes
on the gas conditions in the early Universe require empirical
measurements of the radiation from low-mass black holes.
In practice, since accretion disk emission around supermassive black holes peaks in the ultraviolet, it is difficult to unambiguously measure changes in the disk temperature with mass
(e.g., Davis et al. 2007). We have measured a few intriguing properties of the radiation from low-mass black holes, although thus far we have studied only the most luminous of
them. First, they appear to have a very low incidence of jet activity (Greene et al. 2006). Second, we see indirect evidence
that they have hotter accretion disks than their more massive
Seed Black Holes
cousins (Greene & Ho 2007c; Desroches et al. 2009; Ludwig
et al. 2012; Dong et al. 2012) as expected from basic disk
models (Done et al. 2012). As the accretion disk gets hotter,
its impact on the surrounding gas will grow. Thus, growing
black holes may well impact the formation of the first stars
and galaxies (e.g., Jeon et al. 2012).
Accretion onto a central black hole has been found in lowmass galaxies of all shapes and with all levels of ongoing star
formation. Miller et al. (2012) have found accreting black
holes in galaxies comprised predominantly of old stars (see
also Pellegrini 2010), while Izotov & Thuan (2008), Reines
et al. (2011), and Jia et al. (2011) have reported evidence
of accreting black holes in vigorously star forming galaxies.
Some host galaxies are round (Barth et al. 2004), while others are pure disks (Filippenko & Ho 2003). We display the
variety of host galaxy morphologies in Figure 3.
Supermassive black holes in bulge-dominated galaxies
obey remarkably tight correlations between black hole mass
and the properties of the host galaxy. For a long time it was
simply unknown whether the correlations seen for bulges apply to disk-dominated galaxies; dynamical black hole mass
measurements in disk galaxies are severely compromised by
the presence of dust and young stellar populations. Early on,
based on very indirect arguments, we saw evidence that the relationship between black holes and galaxies extended to lowmass and even bulgeless systems (Barth et al. 2005). However, as the number of available dynamical black hole masses
in disk galaxies grows, it becomes increasingly clear that diskdominated galaxies do not obey tight scaling relations with the
central supermassive black hole (Hu 2008; Greene et al. 2010;
Kormendy et al. 2011). Apparently the physical process that
builds galaxy bulges (the merging of galaxies, we think), is
also instrumental in growing black holes and establishing the
scaling relations between black holes and bulges (e.g., Mihos
& Hernquist 1996).
Let us now determine whether existing observations of lowmass black holes favor a particular formation route for primordial seed black holes. I want to estimate the fraction of galaxies containing black holes as a function of galaxy mass: the
occupation fraction. Based on previous work, we assume that
all galaxies with stellar mass Mgal > 1010 M contain black
holes. To study the occupation fraction in lower-mass galaxies, I will use the two X-ray studies discussed above from
Desroches & Ho (2009) and Gallo et al. (2010) combined
with Miller et al. (2012). Note that existing optical studies,
while they include a larger number of objects, cannot be used
for measuring occupation fractions because of their bias towards luminous host galaxies. While the samples with X-ray
measurements are smaller, they should be unbiased.
Since we are using accretion to discover black holes (via
X-ray emission), we will not detect all black holes. For instance, we may detect X-rays from 10% of galaxies with
Mgal = 1011 M , while they all contain supermassive black
holes. If we then detect X-rays from 1% of galaxies with
Mgal = 108 M , we can conclude that only 10% of Mgal =
108 M contain black holes2 . For each X-ray sample we need
2 We are making an assumption that the fraction of active black holes is
always the same (in my example 10%), independent of the galaxy or the black
hole mass. This is a strong assumption that we must make in order to proceed,
but in detail it is probably incorrect. For instance, galaxies with less gas may
be less likely to harbor an active black hole
a comparison sample of more massive galaxies with similar
X-ray observations. The Gallo et al. sample spans a wide
range in stellar mass, so the comparison sample of bulges is
built in. As a complementary sample of more massive disk
galaxies to compare with the Desroches sample, I take the
archival X-ray survey of Zhang et al. (2009). Given the existing X-ray surveys, we are not sensitive to black holes with
masses MBH . 3 × 105 M .
The only additional complication is that lower-mass black
holes are intrinsically more difficult to detect; their maximum
luminosity gets lower as the black hole mass goes down. In
other words, the Eddington limit (where the radiation pushing
out balances the pull of gravity) increases linearly with black
hole mass. At the luminosity limit of my survey, I can see
massive black holes to a much lower fraction of their maximum luminosity than low-mass black holes. To remove this
bias, I will normalize all the observed X-ray luminosities to
the Eddington limit of that black hole (LX /LEdd ), and then
only consider detections down to a fixed LX /LEdd limit. In
most cases I do not have direct measurements of black hole
mass. Galaxies with Mgal > 1010 M are assigned a black
hole mass based on the correlation between galaxy mass and
black hole mass, while I assume no such correlation holds for
galaxies with Mgal < 1010 M . Instead, I just assume that every black hole has a mass between 3 × 105 and 106 M . The
limiting LX /LEdd is calculated accordingly in each case.
The results are shown in Figure 2. The Desroches & Ho
limits are shown in blue, while the Gallo points are shown
in red, for 106 M (lower) and 3 × 105 M black holes (upper). We have no constraints on black holes of lower mass
yet. The expectations from theory for seeds created via stellar death (green dashed) and direct collapse (solid purple) are
shown as well. Obviously, the limits are not yet good enough
to say anything definitive, but tentatively the data seem to prefer massive seed models.
Improving these limits will require a multi-pronged approach. As argued above, we are reaching the limit of utility
of X-ray surveys because we run into confusion from stellarmass black holes. However, X-ray surveys that stare at the
same part of the sky for a very long time are starting to uncover accreting black holes in galaxies with Mgal < 109 M .
At the same time, combining sensitive radio and X-ray surveys may yield interesting new constraints on black holes at
much lower fractions of their Eddington luminosity.
We focus here on black holes found at galaxy centers. However, we want to highlight two other very interesting locations
that may harbor hitherto unknown low-mass black hole populations: the centers of dense stellar clusters, and in the outer
parts of galaxies.
Another route to seed black hole growth may occur if the
original seed is formed in the center of a dense cluster of
stars. The black hole might grow to 104 − 106 M by accreting smaller black holes at the centers of dense stellar clusters (e.g., Ebisuzaki et al. 2001). Then we may expect to
find ∼ 104 M black holes at the centers of globular clusters. These dense clusters of stars comprise some of the oldest stellar systems in the universe. Despite many searches
3 To calculate uniform stellar mass for all galaxies in both samples, I follow
Gallo et al. (2010) and use correlations between galaxy B − V color and the
mass-to-light ratio of the stars from Bell et al. (2003).
for black holes in stellar clusters, both using dynamical techniques (e.g., van der Marel et al. 2002; Gerssen et al. 2003)
and looking for signatures of accretion (e.g., Maccarone et al.
2005), there is not yet definitive evidence for black holes in
globular clusters.
There is one exception. The most massive globular clusters,
we believe, were formed not as isolated clusters of stars but
rather as the nucleus of a galaxy that was then torn to shreds as
it was eaten by a larger galaxy. These most massive clusters
also tend to show a wider range of stellar age and chemical
compositions than is seen in typical globular clusters, suggesting that they were formed gradually rather than as a single
unit. The three prominent stripped galaxy candidates that are
in our own local neighborhood all show dynamical evidence
for a central 104 M black hole (Gebhardt et al. 2005; Noyola et al. 2010; Ibata et al. 2009), although the detections are
still controversial (van der Marel & Anderson 2010). Radiation from these putative black holes has not yet been detected
(Wrobel et al. 2011; Miller-Jones et al. 2012).
Another suggestive line of evidence for black holes in
the centers of star clusters come from the intriguing “Ultraluminous” X-ray sources. As the name implies, these targets
have very high X-ray luminosities, so high that they exceed
the maximum (Eddington) luminosity for a stellar-mass black
hole. An easy way to explain the high luminosities is to power
these X-ray sources with intermediate-mass black holes with
masses of 100 − 10, 000 M . The evidence is not ironclad,
however, since it is possible to reproduce the properties of
ULXs with stellar-mass black holes in all but a few extreme
cases (e.g., Socrates & Davis 2006). Unfortunately, determining the masses of black holes that power ULXs has proven
impossible to date.
There is a spectacular ULX that deserves mention. ESO
243-49 HLX-1 has an X-ray luminosity greater than 1042 , the
Eddington luminosity for a 105 M black hole (Farrell et al.
2009). The X-ray source is found offset from the main body
of the galaxy ESO 243-49, but at the same distance as the
galaxy (Wiersema et al. 2010). HLX-1 is likely embedded
in a stellar cluster with Mgal ≈ 106 M (Farrell et al. 2012),
perhaps the remnant of a galaxy that was eaten by ESO 24349 in the past. ESO 243-49 HLX-1 is an intriguing source,
but so far no other sources like it are known.
It is quite possible that many intermediate-mass black holes
may reside outside of galaxy nuclei. As galaxies merge, they
acquire black holes as well as stars. Many of these black holes
may never reach the galaxy center, but reside in galaxy halos
(e.g., Islam et al. 2003), where they would be very difficult
to find. If the black hole is massive enough to sink to the
galaxy center, then it may merge with an existing black hole;
the resulting gravitational radiation could in principle eject
the black hole from the galaxy (e.g., Merritt et al. 2004). At
higher redshift, the number of infalling accreting black holes
is high (Comerford et al. 2009), while at low redshift, systems
like ESO 243-49 HLX-1 appear to be rare.
How can we make progress on determining the space density of the lowest-mass black holes? At the moment, we
are limited by the largest distances that we can probe in unbiased samples. Optical spectroscopic surveys such as the
SDSS have yielded large samples, but with selection biases
that are difficult to quantify. Searches in other wavebands,
while cleaner in terms of selection effects, reach limited dis-
tances and thus contain small numbers of objects. I see multiple paths forward. The first is to look harder for black holes
in local galaxies. We are reaching a fundamental limit in using X-rays, since stellar-mass black holes will dominate the
emission in surveys that push an order of magnitude deeper
(Gallo et al. 2010). On the other hand, the increased sensitivity of radio telescopes (particularly the Jansky Very Large
Array; Jansky VLA) open the possibility of a combined radio
and X-ray survey. While on an object-by-object basis there
still may be complications (e.g., Miller-Jones et al. 2012), the
combination of a radio and X-ray source will be compelling
evidence for a low-mass black hole. Likewise, we may be able
to rely on variability (both in the X-ray and in the optical) in
the future (Kamizasa et al. 2012).
The second path is to try to find more accreting black holes
in small galaxies by searching over larger distances rather
than by looking for fainter sources. Very sensitive X-ray surveys (Xue et al. 2011) should allow such experiments, while
the newly refurbished Jansky VLA could perform a very sensitive search using radio wavelengths. We also need better
measurements of what fraction of more massive galaxies contain accreting black holes, as our comparison sample (e.g.,
Goulding et al. 2010).
Thirdly, even if a black hole is completely inactive, and
thus undetectable by most of the methods discussed here, every once in a long while a star will wander too close to the
event horizon and get disrupted. Many likely tidal disruption
events have been observed (e.g., Gezari et al. 2012; Bloom
et al. 2011), likely from stars falling into ∼ 106 − 107 M
black holes. Since tidal disruption events are rare, with at
most one every 105 years per galaxy expected (Magorrian
& Tremaine 1999), we must monitor many galaxies every
year to detect tidal disruption events. Ongoing and upcoming projects are designed to look at the same part of the sky
again and again over years; with these surveys we can hope to
detect many tidal disruptions per year, even around low-mass
black holes (Strubbe & Quataert 2009). Eventually, as the surveys progress, we may be able to use the detection rate of tidal
disruptions in small galaxies as a indicator of the occupation
fraction. There are many surprises still to come.
I thank A. J. Barth, E. Gallo, L. C. Ho, B. Miller, A. Reines,
A. Seth, and M. Volonteri for useful comments.
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