The University Record, January 14, 1997

U-M computer model detects 'fingerprint' of massive black holes in three nearby galaxies

 

Karl Gebhardt, post-doctoral fellow (left), and Doug Richstone, professor of astronomy (right), review data from a computer model Gebhardt developed to detect the unique star velocity pattern produced by a black hole in the center of a galaxy. The model has already detected three new black holes and will be used by Richstone and other astronomers to conduct a systematic search for black holes in other galaxies. The U-M research project was featured in a press conference held Jan. 13 at the American Astronomical Society meeting in Toronto.

Photo by Bob Kalmbach


By Sally Pobojewski
News and Information Services

U-M astronomers have detected a unique star velocity pattern or "celestial fingerprint" indicating the existence of previously undetected massive black holes hidden in the centers of three nearby galaxies.

The black hole "fingerprint" was detected with a new computer model---developed by Karl Gebhardt, a post-doctoral fellow---to analyze observational data from the Hubble Space Telescope and ground-based spectroscopic data.

According to Douglas Richstone, professor of astronomy and leader of a team of 12 astronomers, the model will be used to conduct a systematic search for additional new black holes in nearby galaxies.

"We believe a massive black hole exists at the center of nearly every galaxy in the universe," Richstone says. "Now, with this new computer model, we can try to prove it. Over the next three years, we plan to observe up to 100 galaxies looking for the specific signal of a black hole."

Richstone, Gebhardt and their colleagues announced the discovery of three new black holes and their plans for a black hole census at a press conference held Jan. 13 in Toronto during the American Astronomical Society Meeting.

The new U-M computer model is an improvement over previous modeling

techniques, according to Gebhardt, because it is based directly on observational data, rather than a set of assumptions on how the gravitational attraction of black holes "should" affect the movement of nearby stars. "We measure specific changes in the velocity distribution as you move toward the galactic center---how many stars are at velocity A, how many at velocity B, etc.," Gebhardt explains. "This allows us to squeeze the maximum amount of information possible from a given set of observational data, and detect the sudden change in the velocity pattern which occurs as you move toward the center of a galaxy with a black hole."

"Based on the size of the galaxy and the velocity pattern of stars at the core of the galaxy, we not only can detect the signal from a black hole, we also can predict its mass," Richstone says. "Masses of the three black holes detected with the model so far range from 50 million to 500 million suns."

The galaxies NGC 3379, NGC 3377 and NGC 4486b are all located within 50 million light-years from Earth. The first two galaxies are in the constellation Leo; the third is in Virgo. According to Gebhardt, these three galaxies were selected for the study because previous observations had caused astronomers to suspect they contained a black hole, and because their nearby location made it possible for the Hubble Space Telescope to observe their cores in great detail.

While these three elliptical galaxies are rather ordinary and undistinguished now, Richstone believes energy from stars and gas being pulled into their cores may once have powered quasars---extremely powerful, bright objects capable of generating the luminosity of one trillion suns within an area the size of Mars' orbit. While quasars were common in the early years of our universe's existence, they are now rare. According to Richstone, the black holes lurking within NGC 3379, 3377 and 4486b may be "fossilized quasars," which billions of years ago exhausted their energy source and are now dark and quiet.

Other astronomers involved in the research project include John Magorrian and Scott Tremaine, University of Toronto; John Kormendy, University of Hawaii; Tod Lauer and Ed Ajhar, National Oceanic and Atmospheric Observatories (NOAO); Alan Dressler, Carnegie Observatories; Sandra Faber, University of California; Ralf Bender, Ludwig Maximilian University in Munich; and Carl Grillmair, Jet Propulsion Laboratory.

Research funding was provided by NASA and the Space Telescope Science Institute.