BLACK HOLES IN THE LOCAL UNIVERSE -
Black Holes with Masses exceeding one Million Solar Masses
Max Camenzind
Looking back on the past 15 years, we are struck by how fundamentally astronomers have changed their views of black holes.
In the mid-1980s, AGN research was thriving; the engines that power this activity were believed to be black holes,
although an observational test
- the dynamical detection of black holes - was a high priority.
However, all of this work was very disconnected from other research on
galactic structure and formation. Now we believe that supermassive black holes are standard equipment in galaxies with bulges.
Black holes affect galactic structure in a variety of ways, and they play an important role in galaxy formation.
They are no longer uncertain oddities that
are postulated only to explain some spectacular but specialized observations.
Instead, black holes have become a necessary ingredient in our
understanding of galaxies. We live in exciting times. Continuing HST searches are finding more and more black holes.
By studying them, our
understanding of galaxy formation and evolution should improve rapidly in the coming years.
Black Holes as Engines in Centers of Galaxies
Black holes with masses of a million to a few billion times the mass of the Sun are believed to be the engines
that power nuclear activity in galaxies. Active nuclei range from faint, compact radio sources like that in M31
to quasars like 3C 273 that are brighter than the whole galaxy
in which they live. Some nuclei fire jets of energetic particles millions of light years into space. Almost all astronomers believe that this
enormous outpouring of energy comes from the death throes of stars and gas that are falling into the central black hole. This is a very
successful explanation of the observations, but until recently, it was seriously incomplete: we had no direct evidence that supermassive black
holes exist.
A giant black hole in a galactic nucleus exerts a powerful gravitational force on nearby gas and stars, causing them to move at high speeds.
This is hard to see in quasars, because they are far away and because the
dazzling
light of the active nucleus swamps the light from the host galaxy. In a
radio galaxy with a fainter nucleus, the stars and gas are more visible.
The giant elliptical galaxy Messier 87, one of the two
brightest objects in the Virgo cluster of galaxies, is a radio galaxy with a bright jet emerging from its nucleus. It has long been thought to
contain a black hole. Recent observations of Messier 87 with the Hubble Space Telescope (HST) reveal a disk of gas 500 light years in
diameter whose orbital speeds imply a central mass of 3 billion solar masses. The ratio of this mass to the central light output is
more than 100 times the solar value. No normal population of stars has such a high mass-to-light ratio. This is consistent with the presence of
a black hole, but it does not rule out some other concentration of underluminous matter.
A more compelling argument is possible in the Seyfert galaxy NGC 4258, where
orbiting gas in the nucleus emits microwave maser emission from water
molecules. The location and velocity of this gas can be mapped with amazing
precision by making coordinated observations with radio telescopes separated
by large distances. The angular resolution given by this technique is 100
times better than that of HST. The measurements imply that 40 million solar
masses lie within half a light year of the center. Could this material be a
cluster of dark stars? Dan Maoz (Tel-Aviv University) has shown that
the answer is ``no.'' There are two possibilities, failed stars or dead stars.
Failed stars are ones that are too low in mass; their insides never get hot
enough to ignite the nuclear reactions that power stars. They are called brown
dwarfs. But brown dwarfs are light - less than 0.08 solar masses - so there
would have to be many of them to explain the dark mass in NGC 4258. Then they
would have to be very close together. As a result, most of them would collide
with other brown dwarfs. Stars that collide generally stick together. But if
two brown dwarfs of almost 0.08 solar masses merge, they become a luminous
star, and then the dark cluster would light up. The other alternative is dead
stars, that is, white dwarf stars, neutron stars, or stellar-mass black holes.
But these are more massive than brown dwarfs, so there would be fewer of them.
The gravitational evolution of clusters of stars is well understood:
individual stars get ejected from the cluster, the remaining cluster
contracts, and the evolution speeds up. Calculations show that a cluster of
dead stars in NGC 4258 would evaporate completely in about 100 million years.
From a cosmic perspective, this is almost no time at all. It is much less than
the age of the galaxy. So the most astrophysically plausible alternatives to a
black hole can be excluded. It is difficult to escape the conclusion that NGC
4258 contains a supermassive black hole.
Black Holes in Nearby Galaxies
Typical quasars have redshifts of 2 or 3 (the highest redshift is now beyond
6, i.e. at a time of less than one Gyear !). A redshift of 2-3 corresponds to a
time when the universe was about 3 Giga-years old, only 1/5 of its present
age. At that time, many large galaxies had luminous active nuclei. Since then,
quasars have mostly died out. Very few are active today. What is left is the
waste mass from the quasar era - black holes with masses of a million to a few
billion solar masses that are starving for fuel. Some of them accrete slowly;
this produces very little light. Many are not active at all. Since quasars were
once found in most large galaxies, we expect that dormant supermassive black
holes should hide in most large galaxies today.
Cosmic time versus redshift for two extreme inflationary (flat) models
for a Hubble constant H_0 = 65 km/s/Mpc:
In a LambdaCDM-model (red curve) the universe is 14 Gyears old,
in a standard cold dark matter model (SCDM, blue curve) however only 9 Gyears.
The latter is now ruled out by many observations.
Astonishingly, our present universe is completely dominated by
(invisible) dark matter and by some silly vacuum energy density !
How can we find these black holes? If they are dormant, they make their presence felt only through their gravitational pull on nearby stars and
gas. When they are surrounded by a disk of stars or gas, this disk spins very rapidly. In fact, the signature of a black hole is that the disk spins
too rapidly for the number of stars that it contains. Without an extra dark object in its middle, the disk would fly apart. More generally, the
orbits of stars in a galactic nucleus are not all aligned in a disk but instead have random orientations. They swarm around like a cluster of bees,
rather than spinning like a flying frisbee. A spectrograph sees light from many thousands of stars at once. Some are approaching us in their
orbits and others are receding. As a result, the stellar absorption lines are broadened by the Doppler shift. Measuring the width of the lines
tells us the random velocities of the stars. From the rotation velocities or the random velocities, the laws of gravity tell us the mass.
As of the year 2001, there is good evidence for supermassive black holes in
more than 35 galaxies (Table 1, Kormendy & Gebhardt, Texas Symp 2000). This is
a considerable improvement, but for compelling statistical arguments the
sample has still to be enlarged to a few hundred galaxies !
First, the amount of mass that one finds in black holes is consistent with
predictions of the waste mass left behind by quasars. Also, the individual
masses of the black holes in Table 1 are consistent with predictions from
quasar energies.
Two new results are fundamental correlations between black hole masses and the properties of their host galaxies.
Galaxies come in two basic types, flat spinning disks (like frisbees) and more nearly spherical bulges that rotate a
little but that mostly are supported by random
motions of stars.
Many galaxies, like our own and the Andromeda galaxy, consist of a bulge
in the middle of a disk. When a galaxy contains only a bulge and not a disk,
it is called an elliptical galaxy. In the following discussion, use of the
term ``bulge'' includes elliptical galaxies. Supermassive black holes have now
been found in elliptical galaxies and in galaxies that contain both a bulge
and a disk but not in galaxies that consist only of a disk. In 1993, Kormendy
found that black hole mass is roughly proportional to the luminosity of the
bulge component of the host galaxy. This is confirmed by the new black hole
detections. It implies that the mass of a black hole is always
about 0.2 % of the mass of the bulge. The cause of this correlation is not
known, but it implies that, as galaxies form, an approximately standard
fraction of the mass ends up in the black hole. The correlation contains
important clues to the origin and growth of galaxies.
In 2000, a new and more fundamental correlation has been found by Karl Gebhardt
(now at the University of Texas at Austin) and collaborators, including
Kormendy. More massive black holes live in galaxies whose stars move faster
(Figure: Kormendy). Of course, the stars near the center must have high
velocities; they are the ones that are used to find the black holes
(see Figure on Andromeda and NGC 3115).
Concluding that black holes correlate with these stars would be circular
reasoning. Instead, the new correlation involves the stars in the main bodies
of the galaxies. These stars do not feel the black holes. But they, too, move
more rapidly than do stars in galaxies with less massive black holes. The
scatter in the new correlation is almost zero. That is, it is almost the same
as the measurement errors. Tight correlations in astronomy have always led to
fundamental advances in our understanding of how things work. They tell us
that there is an underlying astrophysical constraint that we didn't know about
before. In the present case, we do not yet have an explanation of why the
correlation is so tight. But it implies that there is something almost
magically regular about the process by which black holes are fed and grown.
On the other hand, we can use the correlation without knowing why it exists. The smallness of the scatter and the existence of two
correlations have much to say about when, in relation to their host galaxies, black holes grew.
Black Hole masses have been found to correlate extremely well with the
velocity dispersion of the stars in the bulge of a galaxy (right panel),
much better than with the luminosity of the bulge (left panel).
The left plot shows the correlation between black hole mass and the luminosity of the bulge component of the host galaxy in
units of the total luminosity of the Milky Way. It tells us that more luminous bulges -- which are also more massive bulges --
contain more
massive black holes. Each black hole is represented by one point. Blue points are detections based on the motions of stars.
Green points are
based on the rotation speeds of hot gas disks.
Red points are based on the rotation speeds of cold maser gas disks. The right-hand plot shows
the new correlation between black hole mass and the average random speeds of stars in the host galaxies. It says that bigger black holes live
in galaxies whose stars move faster. The points have the same meaning as in the left plot.
Dispersion and rotation velocity in Andromeda (Kormendy).
Rotational velocity and velocity dispersion in the S0-galaxy NGC 3115.
Mass distribution in the center of our Galaxis.
Supermassive Black Holes and Galaxy Formation
Initially the Universe has
been very homogeneous and its main
constituent is some sort of non-baryonic
dark matter. The structures we see in the
observable Universe like stars, galaxies,
galaxy clusters and the inhomogeneous
distribution of the diffuse gas between
galaxies have formed by gravitational
instability from small fluctuations in the
matter distribution. These fluctuations can be
directly observed as tiny temperature
fluctuations in the cosmic microwave
background, the relic radiation which is left
over from the time when the Universe was much hotter and denser than today.
As time goes on these fluctuations in the dark matter distribution grow and collapse
into dense dark matter halos . These dark matter halos are assembled into a "cosmic web"of
filamentary and sheet-like structures. The more easily observable baryonic
matter is dragged along by the dark matter. The stars and supermassive black holes which
make up the galaxies we see form from the dense gas within the dark matter
halos while the intergalactic medium in between attains a filamentary and sheet-like
distribution just like the dark matter.
This picture shows a region of about
3 x 3 Mpc (comoving) at z=3 which was
simulated with a numerical technique called
Smoothed Particle Hydrodynamics (SPH). It
contains about twenty protogalactic clumps
with their surrounding intergalactic medium.
The protogalactic clumps which will merge
into a normal galaxy
by the present day are aligned along the
characteristic filamentary and sheet-like
structures typical for gravitational clustering. The interaction between the intergalactic
medium and newly-forming high-redshift galaxies can be studied in great detail even for
very faint galaxies using QSO absorption spectra
So far, astronomers have found a supermassive black hole in every galaxy observed that contains a bulge component.
Therefore the observed correlations say that black hole mass is intimately connected with bulge formation.
Alternative theories come in two extremes.
- Maybe black holes came first in a standard size, namely 0.2 % of the mass of the first galaxy fragments.
Then mergers of small galaxies made big galaxies, and the big galaxies still contained 0.2 % mass black holes because,
when two galaxies merge, their black holes merge, too.
- Maybe black holes started out small and then grew during galaxy formation. If 0.2 % of the gas that makes stars
always gets fed to the central black hole, then the black hole mass fraction is always 0.2 %.
Both theories include an explanation of quasars, but they differ in how they use quasars. In theory (1), the black holes come first and then
regulate galaxy formation, while in theory (2) the black holes and galaxies grow together.
Which theory is correct? Kormendy favors (2).
Observations show that there are two kinds of bulge-like components and that both contain black holes. One kind, called a pseudobulge
(i.e. photometric and kinematic evidence for disklike structures)
believed to form in a bulgeless pure disk galaxy, when gas flows inward
toward the center (this mainly happens in Sc/d galaxies). We observe that disks
do not contain supermassive black holes in nearly the same proportion (0.2 % of
the mass) as do bulges. But seven galaxies in Table 1 contain pseudobulges and
all of them contain standard black holes with about 0.2 % of the pseudobulge
mass. So the black holes must have grown during the process that made the
pseudobulges.
The second argument comes from a comparison of the two correlations of black hole mass with host galaxy properties. One correlation says
that bigger bulges contain bigger black holes, with exceptions: a few galaxies contain anomalously big black holes. But the stars in these
galaxies move faster, too, and they do so by precisely the right amount so that the scatter in the black hole mass -- random velocity
correlation is small. What does this mean? The reason why the stars move so rapidly is that the galaxy collapsed to an unusually small size
when it formed. Then stars are closer together, so their gravitational forces on each other are bigger, so they must move faster. But if black
holes are unusually massive whenever galaxies are unusually collapsed, then black hole masses was probably fixed by the collapse process.
The alternative -- that bigger black holes cause a galaxy to be more collapsed -- is less likely, because bigger black holes would power
brighter quasars; their radiation would push on the protogalactic gas and would tend to make it collapse less, not more.
Based on these arguments, we conclude that the major events that made the bulge and the major periods of black hole growth were the same
events. Galaxy formation directly results in the black hole feeding that makes quasars shine.
The idea that bulge formation, black hole growth, and quasars happen together is not new, but it has been only one of several competing
theories. In 1988, David Sanders (now at the University of Hawaii) and a group of colleagues suggested that a rare type of galaxy that is
extremely luminous in infrared light is an early stage in the development of quasars. These ``ultraluminous infrared galaxies'' were quickly
shown to be galaxy mergers that are making bulges. They are our best local examples of how bulges formed. But are they making quasars?
The idea was attractive but controversial. It led to a decade-long debate about whether ultraluminous infrared galaxies are powered by
active nuclei or by starbursts. Recent observations suggest that both sides are correct: about 2/3 of the energy comes from starbursts and
about 1/3 comes from nuclear activity. This is just what the present picture requires. HST observations of quasar host galaxies also add
support, because many are disturbed systems and plausible mergers in progress. Recent submillimeter detections of distant galaxies that are
similar to ultraluminous infrared galaxies have been interpreted by some astronomers as the discovery of bulges in formation.