February 13, 2001

Galaxies and black holes: You can't have one without the other

EDITORS: Color images of a galaxy containing a black hole with mass equivalent to 100 million suns are available for electronic download at: http://www.astro.lsa.umich.edu/users/dor/n4697c.jpg and at: http://www.astro.lsa.umich.edu/users/dor/xvsnap.jpg

SAN FRANCISCO—Galaxies and black holes are so intimately connected that it is almost impossible to find one without the other, according to University of Michigan astronomer Douglas Richstone.

Over the last decade, Richstone and a team of researchers have detected massive black holes in all but one of the 30 spiral galaxies they surveyed. To detect new black holes, scientists look for abrupt changes in star velocity patterns revealed by stars orbiting near the center of the galaxy. Based on the galaxy's size and the velocity pattern of stars at the galaxy's core, scientists can detect the gravitational force of a black hole and also estimate its mass.

"The mass of these objects appears to correlate with the mass of the central part of their host galaxy," said Richstone, a U-M 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. Black holes and stars compete for baryons, or particles of matter, that form stars during the early life of galaxies."

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 we now see in centers of galaxies are relics of these quasars," Richstone explained. "So black holes must have already been present at the height of the quasar epoch when the universe was about one billion years old."

Even though black holes make up only two-tenths of one percent of a galaxy's mass, their energy efficiency is 10 times greater per unit of mass than all the stars in the galaxy, according to Richstone. The thermal and mechanical luminosity of these early black holes dominates the energy output of young stars forming in the proto-galaxy.

"The energy output of a massive black hole is comparable to the energy of the galaxy over its entire lifetime," Richstone said. "Black holes and stars compete for mass, and black holes heavily influence the thermodynamics of the interstellar gas in young galaxies. So it is impossible to sensibly discuss the galaxy formation process, or the history of star formation, without including the formation of black holes."

Other astronomers on Richstone's black hole research team include: Ralf Bender of the Ludwig Maximillian University in Munich; Gary Bower, Richard Green and Tod Lauer of National Optical Astronomy Observatories; Alan Dressler and Luis Ho of Carnegie Observatories; Sandra Faber of the University of California-Santa Cruz; Alex Filippenko of the University of California-Berkeley; Karl Gebhardt of the University of Texas; Carl Grillmair of SIRTF Science Center; John Magorrian of the University of Durham, United Kingdom; Jason Pinkney of the University of Michigan; and Scott Tremaine of Princeton University.

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

Contact: Sally Pobojewski
Phone: (734) 647-1844
E-mail: pobo@umich.edu