June 7, 2000 Astronomers are concluding that monstrous black holes weren't simply born big but instead grew on a measured diet of gas and stars controlled by their host galaxies in the formative years of the universe. These results, gleaned from a NASA Hubble Space Telescope census of more than 30 galaxies with its powerful "black hole hunting" spectrograph, are painting a broad picture of a galaxy's evolution and its long and intimate relationship with its giant central black hole.
Though much more analysis remains, an initial look at Hubble evidence favors the idea that titanic black holes did not precede a galaxy's birth but instead evolved with the galaxy by trapping an amazingly exact percentage (0.2 percent) of the mass of the bulbous hub of stars and gas in a galaxy.
This means that black holes in small galaxies went relatively undernourished, weighing in at a mere few million solar masses. Black holes in the centers of giant galaxies, some tipping the scale at over one billion solar masses, were so engorged with infalling gas they once blazed as quasars, the brightest objects in the cosmos.
The bottom line is that the final mass of a black hole is not primordial; it is determined during the galaxy formation process. "This supports the original theory of why black holes are important and how they got their masses. It suggests that the major events that made a galaxy and the ones that made its black hole shine as a quasar were the same events," says John Kormendy of the University of Texas at Austin. "These results are a catalyst that helps to tie together many lines of investigation."
These results are being reported at the 196th meeting of the American Astronomical Society in Rochester, NY, by Kormendy, Karl Gebhardt (Lick Observatory), Douglas Richstone (University of Michigan) and an international team of collaborators.
Though this secret relationship between a black hole and its host galaxy has been suspected for the past several years, it is bolstered by the Hubble discovery of 10 more super massive black holes in galaxy centers, raising the total to more than 30 black holes now available for study. "For the first time we can put strong constraints on the relationship between galaxy formation and black hole formation and growth," says Kormendy.
The results show a close relationship between the black hole mass and the stars that comprise an elliptical galaxy or the central bulge stars of a spiral galaxy. But surprisingly, an even tighter correlation is found. In most cases the black holes not only bulked up through the accretion of gas in isolated galaxies, but also through the mergers of galaxies where pairs of black holes combined.
The results also explain why galaxies with small bulges, like our Milky Way, have diminutive central black holes of a few million solar masses, while giant elliptical galaxies house billion-solar-mass black holes, some still smoldering from their days as quasars. Disk galaxies without a central bulge of stars either have no black hole or have only tiny black holes that are well below Hubble's detection limit.
The findings are based on two types of Hubble observations. Several teams measured the black holes' masses by recording the whirling speeds of disks of gas trapped around the black holes, like water swirling around a drain. Other teams measured the motions of stars around the galaxies' hubs like a swarm of bees hovering around a beehive. The more massive the bulge, the greater the speed of the stars.
"Other observations of the entire stellar mass of the bulge show a very tight relationship between a black hole's mass and the depth of the gravitational potential well as measured by the magnitude of random velocities of stars in the galaxy's hub. This bolsters the conclusion that the mass correlation is real," says Gebhardt.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
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