June 5, 2000

CONTACT: John Kormendy Department of Astronomy RLM 15.308 University of Texas at Austin Austin, TX 78712 Phone: (512) 471 - 8191 Email: Karl Gebhardt Lick Observatory University of Calfornia at Santa Cruz Santa Cruz, CA Phone: (831) 459 - 3841 Email: Douglas Richstone Department of Astronomy University of Michigan Ann Arbor, MI 48109 Phone: (734) 764 - 3440 Email:

Astronomers announce the discovery of 8 supermassive black holes in galaxy centers. The 33 black holes now available for study reveal a fundamental new correlation between black hole mass and galaxy formation as measured by the random velocities of stars. Black holes masses are closely connected with the properties of the dense, elliptical-galaxy-like ``bulge'' parts of galaxies. In contrast, they are not connected with galaxy disks: pure disk galaxies do not contain supermassive black holes. These results suggest that black holes grew to their present sizes as part of the galaxy formation process. That is, the growth of black holes -- when they shone brightly as quasars -- was fed by the same inward collapse of gas that made the stars in galaxies. These conclusions tie together a variety of observations and theories into a newly coherent picture of galaxy formation and black hole growth.

These results are being presented on Tuesday, June 6th, 2000 at the 196th Meeting of the American Astronomical Society in Rochester, NY, by John Kormendy of the University of Texas at Austin, Karl Gebhardt of the University of California at Santa Cruz, Douglas Richstone of the University of Michigan, and an international team of collaborators.


The Hubble Space Telescope and Space Telescope Imaging Spectrograph (STIS) are uniquely efficient tools for finding black holes. The pace of discoveries is accelerating: the present team announces 8 new black hole detections and other teams here add several more. Also, analysis techniques have improved, so measurements are more accurate. At least 33 black hole candidates are available; a census is given at the end of this report. This is enough data for a quantum improvement in our ability to study the connection between black holes and galaxy formation.


Based on the new and more accurate measurements, the astronomers have discovered a fundamental correlation between the masses of black holes and the average random velocities of the stars in their host galaxies. Karl Gebhardt says, ``More massive black holes live in galaxies whose stars move faster. By this, I don't mean the stars near the center that we use to find the black holes, I mean the stars in the main bodies of the galaxies. These stars don't feel the black holes. They can move fast for two reasons, first if a galaxy is very massive and second if it collapsed more than average when it formed.''

The first aspect of this correlation -- that bigger galaxies contain bigger black holes -- has been reported previously. This correlation is now measured more accurately. A typical black hole ``weighs'' 0.2 % of the mass of the high-density central part -- the ``bulge'' -- of its galaxy.

The second aspect of the correlation is new. Black holes ``know about'' how much their host galaxies collapsed when they formed.

Most important of all, the scatter in the new correlation is very small. Gebhardt notes that ``The scatter is almost zero. In other words, it is almost the same as the measurement errors.'' Team member Sandra Faber emphasizes why this is important: ``Tight correlations in astronomy have always led to fundamental advances in our understanding of how things work. Finding the answers can take decades of work, but a tight correlation tells us that there is an underlying astrophysical constraint that we didn't know about before.''. In the present case, the astronomers 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, astronomers 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.


In sharp contrast to the close connection between black holes and bulges, John Kormendy notes that ``Black holes do not correlate with galaxy disks at all. In fact, pure disk galaxies -- ones that don't have a bulge component -- apparently don't have supermassive black holes, either.'' An example of a pure disk galaxy is our neighbor, Messier 33. ``If disks contained black holes like bulges do, then Messier 33 should have a black hole of mass 100,000,000 Suns. But the observations show that it cannot contain a black hole more massive than 2000 Suns. Black holes don't know about disks.''


What does all this mean?

Astronomers find supermassive black holes in every galaxy observed that contains a bulge component. Therefore the observed correlations say that black hole mass is intimately connected with bulge formation. There are two alternative theories. (1) 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. Or (2) 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. Quasars are the brightest galactic nuclei known. Many outshine the galaxies that they live in. More than 30 years ago, supermassive black holes were postulated as the engines that make quasars shine. The black holes were thought to swallow nearby gas and stars, which radiate ferociously on the way into the hole as they get accelerated to almost the speed of light. This picture predicted the masses of black holes, and it was the reason why astronomers looked for black holes in the first place. However, in theory (1), above, the black holes are formed first and then regulate galaxy formation, while in theory (2) the black holes and galaxies grow together.

Which theory is correct? Two arguments favor (2). The first is based on the observation that black hole mass does not correlate with disk mass.

(A) Observations show that there are two kinds of bulge-like components and that both contain black holes. One kind, called a ``pseudobulge'', is believed to form in a (bulgeless) pure disk galaxy when gas flows inward toward the center. Disks do not contain supermassive central black holes. Yet pseudobulges that form from disks contain black holes with the standard 0.2 % of the pseudobulge mass. So the black holes must have grown during the process that made the pseudobulges.

(B) The two correlations of black hole mass with host galaxy properties provide the second argument. 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 must be fixed by the collapse process. The alternative -- that bigger black holes cause a galaxy to be more collapsed -- is unlikely, because bigger black holes would power brighter quasars; their radiation would push on the protogalactic gas and make it collapse less, not more.

The astronomers conclude, as Kormendy says, 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 aspects of it have been controversial. In 1988, a different group of astronomers 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. The idea that they are also making quasars provoked great interest but was controversial. It led to a 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. Hubble Space Telescope 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. The present results add strong support to all of these conclusions. The developing interconnection between black holes and other work on galaxy formation is one reason why the combined picture is compelling.


(1) New detections bring the total number of black holes available for study to a large enough number so that astrophysical questions about galaxy formation can be addressed.

(2) A new correlation between black hole mass and the magnitude of random motions in galaxies has been found. It has remarkably little scatter and so puts strong constraints on galaxy formation theories.

(3) Black holes correlate with bulge properties, but they do not correlate with galaxy disks. The few pure disk galaxies that have been observed do not contain supermassive black holes.

(4) Astronomers can now choose between the two alternatives ``black holes came first'' and ``black holes grew and shone as quasars as part of the galaxy formation process''. The observations support the second alternative.

(5) The new observations are a catalyst that ties together many lines of investigation on galaxy formation into a more believable and coherent picture.

Kormendy says, ``Bulges appear to form early in violent collapses that make lots of fireworks: starbursts that feed black holes and make quasars. Disk formation is wimpy: disks form gently and slowly, and as far as we know, they don't make black holes. The close connection between black holes and bulges and the complete absence of any connection between black holes and disks emphasizes how different the two galaxy formation processes really are.''

Richstone adds: ``The observed correlations of black holes with galaxy properties are solid. Interpretation is harder. Galaxy formation was complicated and our observations of faraway things are incomplete. But the connection between black holes and galaxy formation is now clearing up rapidly.''


Where did the seed black holes come from that grew monstrous during the quasar era? Small primordial seed black holes could have formed early in the history of the universe and helped to promote galaxy formation. The present argument is only that their masses must have been small compared to the masses that got added during the merging and quasar era. Alternatively, small seed black holes could have formed at the beginning of galaxy formation and immediately started to grow by swallowing the surrounding gas. Richstone emphasizes that ``Unlike stellar-mass black holes, which are known to form when massive stars die, the formation mechanism for the seed black holes that grew rapidly during galaxy formation remains a mystery. Our team will try to solve this mystery in the coming year by looking for the smallest nuclear black holes that we can find with Hubble.''


The ``Nuker Team'' reporting this work consists of Prof. Ralf Bender (Ludwig Maximilian University, Munich, Germany), Dr. Gary Bower (National Optical Astronomy Observatories), Dr. Alan Dressler (Carnegie Observatories), Prof. Sandra Faber (University of California at Santa Cruz), Prof. Alex Filippenko (University of California at Berkeley), Dr. Karl Gebhardt (University of California at Santa Cruz), Dr. Richard Green (National Optical Astronomy Observatories), Dr. Carl Grillmair (California Institute of Technology), Dr. Luis Ho (Carnegie Observatories), Prof. John Kormendy (University of Texas at Austin), Dr. Tod Lauer (National Optical Astronomy Observatories), Dr. John Magorrian (Cambridge University, England), Dr. Jason Pinkney (University of Michigan), Prof. Douglas Richstone (University of Michigan, Team Leader), and Prof. Scott Tremaine (Princeton University).


The Nuker team is supported by HST data analysis funds through grants GO-02600.01-87A and GO-07388.01-96A. Karl Gebhardt is supported by NASA through Hubble Fellowship grant HF-01090.01-97A awarded by STScI. John Kormendy is Curtis T. Vaughan, Jr. Centennial Chair in astronomy at the University of Texas; he is grateful for financial support from this position. Douglas Richstone is supported by NASA grant NAG-8238.


Black Hole Detection: Black holes are never observed directly: they are too small, and only the material that surrounds them can emit light. Instead, astronomers observe how fast the stars near galaxy centers are moving. They find that the stars move so rapidly that there must be more mass pulling on them than the gravity of the stars can explain. That is, dark masses of a million to a billion times the mass of our Sun must be present in galaxy centers. The inference that these dark masses are black holes is indirect; for example, in two galaxies, the dark objects are so dense that plausible black hole alternatives are eliminated.

Bulges and Disks: Galaxies come in two basic types, thin spinning disks and more nearly spherical ``bulges'' that rotate a little but that mostly are supported by random motions of stars. Many galaxies consist of a bulge in the middle of a disk. When a galaxy contains only a bulge and not a disk, astronomers call it an elliptical galaxy. In the above 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.