Press Release

Astronomer says black holes influenced galactic evolution

By SpaceRef Editor
January 10, 2000
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News Release: January 10, 2000 (9)

Astronomer says black holes influenced galactic evolution

ATLANTA — 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 news conference held here Jan. 13 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 Douglas Richstone, leader of the research team and a University of Michigan
professor of astronomy. “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 says 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 University of Michigan,
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 (NOAO); 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.

SpaceRef staff editor.