December 12, 2000

CONTACT: Karl Gebhardt or John Kormendy Department of Astronomy RLM 15.308 University of Texas Austin, TX 78712 Phone: (512) 471 - 1473 (for Gebhardt) or 471 - 8191 (for Kormendy) Email:;

Quasars are believed to be supermassive black holes that are actively swallowing gas and stars. Measuring their masses would provide an enormous improvement in our understanding of their complex physical environment. More importantly, because of their great distances, mass measurements would provide a direct probe of the evolutionary history of the black hole and its host galaxy. Astronomers Karl Gebhardt and John Kormendy of the University of Texas at Austin, together with an international team of collaborators, have now shown that they can provide reliable black hole masses for active galactic nuclei such as quasars.

Direct measurements of supermassive black holes have been made in at least 38 galaxies, based on large rotation and random velocities of stars and gas near their centers. Such measurements require high spatial resolution such as that provided by the Hubble Space Telescope. They are possible only for nearby galaxies. Since quasars are too distant to apply these direct methods, astronomers have relied on physical models of the region near the black hole to measure its mass. These techniques suffer from large and unknown uncertainties because of the complex nature of quasar physics. However, given the recently discovered relationship between black hole mass and galaxy mass seen in the nearby sample, astronomers can now calibrate quasar models. This promises to provide a large increase in their presently small sample of 38 galaxies with black holes. It will be possible to estimate black hole masses for thousands of galaxies out to distances that explore the epoch of galaxy formation. Two techniques for measuring black hole masses in quasars are available, reverberation mapping and photionization models.

Reverberation mapping relies on the variability of quasars, and the fact that there are numerous clouds of gas that orbit near to the supermassive black hole. As the black hole varies its energy output, the brightness of the radiation from the orbiting clouds varies as well. However, since light travels at a finite speed, the brightness variations in the orbiting clouds are seen later than those in the central engine source. The time difference tells astronomers how far the orbiting gas clouds are from the black hole. How fast the clouds are orbiting can also be measured. Together, the results measure the mass of the black hole. However, there has been no way to test the results, and some of the properties of the gas clouds are uncertain.

Photoionization models are even more uncertain, because they depend on an empirical relationship that astronomers are still trying to understand. That is, the amount of radiation that an orbiting cloud emits depends on how far away it is from the black hole. Thus, by simply measuring how "bright" the cloud appears, astronomers can deduce its radius from the center. Knowing both the radius and the speed, astronomers can then measure the black hole masses. Until now, astronomers were reluctant to trust masses from either of these techniques. The following plot shows that both are quite reliable.

This figure shows that more massive black holes are found in galaxies in which the stars move faster. The average star speed is the random velocity of stars at large radii where they cannot gravitationally feel the black hole. The black points are accurate black hole masses based on high-resolution observations of (circles) stellar motions, (triangles) the rotation of hot gas, and (squares) the rotation of cool molecular gas. The green points are based on reverberation mapping and the red points are based on photoionization models. The figure demonstrates that both techniques (red and green) agree with black hole masses from detailed dynamical modeling (black).


Both reverberation mapping and ionization models can be applied even to the most distant quasars. Large surveys are under way that should provide thousands of black hole masses. These will in the coming years make it possible to probe the growth of black holes in the early Universe. Furthermore, the calibration of the quasar techniques provides the detailed information required to probe the structure of the central engine. Therefore astronomers will be able to explore both galaxy formation and black hole growth in exquisite detail.

Background on Black Holes in Nearby Galaxies

This review from Science, 289, 1484 (2000) summarizes the results on supermassive black holes in nearby galaxies that were announced at the Rochester meeting of the American Astronomical Society in June 2000. Since then, four new black hole detections and three strong black hole mass limits have been added. They strengthen but do not change the conclusions.

A more detailed technical review of recent work is available at astro-ph/0007401. General reviews of the black hole search are in Kormendy & Richstone 1995, Annual Review of Astronomy and Astrophysics, 33, 581; Richstone et al. 1998, Nature, 395, A14, and at astro-ph/0003267 and 0003268. A popular-level review written for Stardate, 28, 4 (2000) is available here

The "Nuker" Team

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.