Supermassive Black Holes Reveal New Clues to Galaxy Formation

By McDonald Observatory

June 5, 2000

AUSTIN, Texas: An international team of astronomers, including Dr. John Kormendy of the University of Texas at Austin, has discovered eight new supermassive black holes, revealing important new clues to the process of galaxy formation. The discoveries will bring the total number of black holes so far found in the universe to at least 33. The findings were being announced Tuesday, June 6, at the 196th meeting of the American Astronomical Society in Rochester, N.Y.

The team includes Kormendy, who holds the Curtis T. Vaughan, Jr. Centennial Chair in Astronomy at UT Austin’s Department of Astronomy, Dr. Karl Gebhardt, a Hubble postdoctoral fellow at the University of California at Santa Cruz, Dr. Douglas Richstone, a professor of astronomy at the University of Michigan’s Department of Astronomy, and an international team of collaborators.

“With the new black hole findings, we have left the ‘Gee whiz!’ discovery phase of this subject and entered the phase of doing science with supermassive black holes,” Kormendy said. “In the early days of the search, from the late 1980s through the mid-1990s, all the emphasis was on finding out whether these black holes really exist.”

Black holes are so compressed that their surface gravity is strong enough to keep even light from escaping. Some black holes have masses several times the mass of the Earth’s Sun. These are formed when massive stars die. Supermassive black holes, in contrast, are a million to a billion times the mass of the Sun and are found at the centers of galaxies.

Because black holes are invisible, scientists can detect and study them only by observing the movements and velocities of the stars swirling around them. The first supermassive black hole was discovered in 1984, although astrophysicists for years had been predicting black holes on the basis of their theories of how the galaxies operate.

“Supermassive black holes were predicted by our theory of what powers quasars, the brightest objects in the universe. An enormous amount of work on quasars had been done,” Kormendy said. “But an important piece of the puzzle was missing. Nobody was sure that supermassive black holes really exist.”

Kormendy started his search in 1985 and discovered the second, third, and fourth supermassive black holes while observing at the Canada­France­Hawaii Telescope on Mauna Kea, Hawaii.

Since then, new discoveries have been made at a rate of one or two per year. The discoveries being announced at the Rochester AAS meeting are the result of 18 months of data collected by the new Space Telescope Imaging Spectrograph on the Hubble Space Telescope. In addition, analysis methods have improved and measurements have become more accurate.

Kormendy said that at least six new black holes have been discovered by other scientific teams. “Suddenly the number of black holes available to us has doubled. This has happened, basically, in the last few weeks as people, including our team, rushed to prepare for this meeting,” Kormendy said. “The new spectrograph is a much more efficient way to look for black holes. Many people have been gearing up to find them.”

With 33 black holes available for research, scientists have enough material to develop a much clearer picture of how galaxies form and how black holes grow. For example, the latest discoveries reveal a fundamental new correlation between black hole mass and galaxy formation, as measured by the random velocities of stars.

Galaxies exist in two basic forms, with variations. Kormendy explained that there are disk or Frisbee-shaped galaxies, such as the Milky Way, and denser, rounder galaxies bulging like partially deflated beach balls. Black holes appear to be closely connected with the properties of the elliptical or bulge-shaped galaxies, but pure disk galaxies do not seem to contain supermassive black holes. The properties of black holes suggest that they grow to their present sizes as part of the galaxy formation process.