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
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,"
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
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
"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.