SEATTLE — A study at Ohio State University has uncovered
more evidence that black holes form before the galaxies that
contain them.

The finding could help resolve a long-standing debate, said
Marianne
Vestergaard, a postdoctoral fellow in astronomy
at Ohio State.

Vestergaard came to this conclusion when she studied a collection
of very energetic, active galaxies known as quasars
as they appeared some 12 billion years ago, when the universe
was only one billion years old. While the quasars were obviously
young — they contained large stellar nurseries in which new
stars were forming — each also contained a very massive, fully
formed black hole.

More and more, black holes are being found at the center of
galaxies. As the close relationship between black holes and galaxies
has emerged, astronomers have debated which of the two came first.

One model holds that mass builds up at the center of galaxies,
eventually collapsing so black holes can form. Another holds
the opposite — that black holes exist first, and their immense
gravity draws gas, dust, and stars together, causing galaxies
to form.

Looking at this evidence, I have to think that black holes
start forming before galaxies do, or form at a much faster rate,
or both, Vestergaard said. She described her study January 8
at the American Astronomical Society
meeting
in Seattle.

One year ago, Vestergaard announced that she had developed
a new method for estimating the mass of very distant black holes,
ones that existed far in the past. The method involves comparing
the spectrum of light emitted by the quasars that host the black
holes to spectra from quasars existing today.

Astronomers consider a galaxy active when it emits much more
energy from its nucleus than can be accounted for by its stars
alone. This radiation is detected at wavelengths that span from
radio waves to X-rays, Vestergaard explained.

Quasars are the most energetic of the active galaxies, from
which all the energy spills out of a very small region at the
center, equal to about one-millionth of the diameter of the total
galaxy. It is in these central regions that black holes reside.

For this latest study, Vestergaard used her method to examine
a special set of distant quasars. Part of her data came from
the Sloan Digital Sky Survey,
a collaborative project that maps the universe from Apache
Point Observatory in New Mexico. She compared the spectra
from those quasars to other quasars that are closer to Earth,
including ones documented by the Bright
Quasar Survey.

In the several hundred quasars she studied, a pattern emerged:
even the smallest, most quiescent of these active galaxies contained
a massive black hole, on the order of 100 million times more
massive than our sun.

Theoretically, the black holes should have taken a long time
to grow that big, if they started out as small seed black holes
and grew by accretion alone; yet, their host galaxies showed
ample signs of youth, such as intense star formation, copious
amounts of molecular gas and significant dust production.

This information could help astronomers better understand
active galaxies, as well as more typical inactive galaxies such
as our own.

All these issues are intertwined — the powering of the central
engine of an active galaxy, the forming of black holes, the forming
of galaxies, she said.

She added that future developments in this area will depend
on KRONOS,
a satellite proposed to NASA
by Bradley Peterson, professor of astronomy at Ohio State, and
his partners from around the world. KRONOS will be able to image
material spiraling into black holes with a resolution 10,000
times finer than now possible with the Hubble
Space Telescope.

For instance, how fast do black holes grow? Do they grow only
by accumulating matter from around themselves, or do they also
need some cataclysmic trigger event, such as when two galaxies
collide? We need deep surveys of the universe to answer these
questions, Vestergaard said.

Other pieces of the puzzle will come from researchers such
as Ohio State graduate student Adam Steed, who is working with
astronomy professor David
Weinberg to model black hole growth.

If we could construct a complete model of what happens to
a black hole over its lifetime, we could look at real black holes
from different points in the past, and see whether our model
is consistent, Vestergaard said. That would be really exciting,
and we would understand more about what is happening in the universe
today.

Vestergaard remains optimistic that astronomers can conquer
these hurdles in the near future.

I never thought we would come to a day in my lifetime when
we could measure the mass of such distant black holes, she said.
But here we are.

Marianne Vestergaard, (614) 292-5807; Vestergaard.1@osu.edu
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu