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Black Holes Shed Light on Galaxy Formation

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 early 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 co-evolved with the galaxy by trapping a surprisingly exact percentage (0.2%) 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 that 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 help to tie together many lines of investigation on galaxy formation into a more believable and coherent picture."

These results are being reported at the 196th meeting of the American Astronomical Society in Rochester, New York, 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 supermassive 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 now 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. "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.

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.

"Hierarchical clustering and merging are an integral part of the picture that we advocate, and to the extent that no new stars get formed, they will in any case preserve the correlation between black hole mass and bulge size," says Kormendy. "This theory has the advantage that it also accounts for quasar activity. The black hole feeding that makes the black hole's mass grow is also what makes the quasar shine. A quasar is the brilliant signature of the fueling and building of the central black hole."

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 (like the neighboring galaxy Messier 33) either have no black hole or have only tiny black holes that are well below Hubble's detection limit.

An alternative but less favored idea is that black holes came first, all packaged in a standard size, namely 0.2 percent of the mass of the first galaxy fragments that formed. Then mergers of small galaxies made bigger galaxies, and the standard black hole mass fraction was preserved because, when two galaxies merge, their black holes merge too. This idea is not favored by the new observations.

The results do not shed light on how seed black holes originate. They are just required to be in place early in the galaxy formation process so that they can grow and shine as quasars. Nor do astronomers know why the galaxy formation process makes a black hole with such a precisely correlated mass. Evidently, the process that decides how much mass gets fed to black holes produces almost the same result, largely independent of the details of galaxy formation.

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