The University Record, January 17, 2000

Which came first—black hole or galaxy?

By Sally Pobojewski
News and Information Services

A team of astronomers conducting a systematic search for supermassive black holes has discovered three more of the mysterious objects lurking in the centers of nearby elliptical galaxies. This brings the total number of supermassive black holes definitively identified so far to 20. The discovery was announced at a Jan. 13 news conference in Atlanta during the American Astronomical Society Meeting.

“The formation and evolution of galaxies are intimately connected to the presence of a central massive black hole,” said astronomy Prof. Douglas Richstone, leader of the research team. “Radiation and high-energy particles released by the formation and growth of black holes are the dominant sources of heat and kinetic energy for star-forming gas in protogalaxies.”

Richstone said the team’s conclusions are inferred from two pieces of evidence. First, all or nearly all galaxies with spheroidal distributions of stars (bulges in spirals) seem to have massive black holes. The mass of these objects seems to correlate with the mass of the central part of the host galaxy. “The ubiquity of this association, as well as the correlation, points to a connection between the massive black hole and the galaxy, and poses a ‘chicken and egg’ dilemma of which came first,” Richstone said.

Second, comparisons of the history of star formation in the universe with the history of quasars, conducted by other scientists, reveal that quasars developed well before most star formation in galaxies. Quasars are extremely powerful bright objects capable of generating the luminosity of one trillion suns within a region the size of Mars’ orbit.

“The massive black holes now seen in centers of galaxies are relics of these quasars,” Richstone explained. “So these black holes must have been present at the height of the quasar epoch when the universe was about one billion years old.”

To detect the existence of new black holes, Richstone and his colleagues use a computer model to detect abrupt changes in star velocity patterns as stars spiral closer to the galactic center. Input for the computer model comes from Hubble Space Telescope observations and ground-based spectroscopic data obtained at MDM Observatory, which is operated by the U-M, Dartmouth College, Columbia University and Ohio State University. The technique was developed by Karl Gebhardt, a former U-M postdoctoral fellow, who is now affiliated with Lick Observatory at the University of California, Santa Cruz.

“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 estimate its mass,” Gebhardt said. “Masses of the three new black holes detected with the model range from 50 million to 100 million suns, which continues our previously reported correlation of black hole mass with galaxy mass.”

The team’s newest black holes were identified in galaxies NGC 4473 and NGC 4697, located in the constellation Virgo, and NGC 821 in Aries. “Stellar orbital distributions in these three galaxies are characteristic of others studied with our model, suggesting a common history among all galaxies,” Gebhardt said.

Other astronomers on the black-hole-census research team include Jason Pinkney, a U-M postdoctoral fellow; John Magorrian, Cambridge University; John Kormendy, University of Hawaii; Tod Lauer, Gary Bower and Richard Green, National Optical Astronomy Observatory; Alan Dressler and Luis Ho, Carnegie Observatories; Sandra Faber and Alex Filippenko, University of California; Ralf Bender, Ludwig Maximilian University in Munich; and Scott Tremaine, Princeton University.

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