UCLA team finds evidence of massive black holes around Milky Way center

Twenty-six thousand light years away, at the center of our
galaxy, a band of renegade black holes is congregating like a horde
of bandits, flinging the hapless stars that cross its path into
space, sending some directly into the maw of the massive black hole
in the middle of the Milky Way, and possibly shaking the fabric of
space-time itself.

Last Monday at a meeting of the American Astronomical Society in
San Diego, Michael Muno, a UCLA postdoctoral Hubble Fellow,
presented the first evidence of a massive conglomeration of tens of
thousands of black holes surrounding the colossal black hole at the
center of the Milky Way galaxy.

Mark Morris, co-author of Muno’s report and a UCLA
astronomy professor, says the finding will help scientists learn
how our galaxy, as well as others, evolved.

“Galaxy evolution is apparently tied to the evolution of
the supermassive black hole at the center,” said Morris.
“We don’t yet know all that much about how these black
holes grow.”

Nearly 4 million times the mass of the sun, the supermassive
black hole at the center of the galaxy grows by devouring nearby
stars, including black holes, Morris said. He explained that the
presence of so many black holes near the monstrous central black
hole could play a vital role in how it feeds and grows.

“If there is a cluster of black holes there, that would
alter the rate at which stars get eaten by that (central) black
hole,” Morris said.

With their immense gravitational pull, black holes influence the
orbits of stars in the neighborhood of the center of the galaxy,
kicking them out of reach of the massive central black hole ““
or pushing them closer toward it, Morris said.

According to Muno, even light cannot escape their immense
gravitational pull, forcing his team to infer their presence
indirectly, by searching for “transient” X-ray sources
““ explosions of light bursting at frequencies a hundred times
higher than the human eye can see, and lasting months at a
time.

Muno’s team, which includes UCLA astronomy professor
Andrea Ghez, collected its data using NASA’s Chandra X-ray
Observatory, according to the Harvard University Chandra Web
site.

“Chandra’s much more sensitive than any instrument
we’ve had access to,” Muno said, with images 10 to 30
times sharper than those of other X-ray detectors.

These X-ray bursts are a sign that a super-dense object like a
black hole is closely orbiting another star, Morris said.

In close contact with another star, Morris said, a black hole
will begin to tear its lighter companion to pieces, sucking its
matter into a hot, spiralling, disk-shaped vortex surrounding the
black hole. He added that the disk of abducted material is highly
unstable and can crash into the black hole suddenly, sending out
its brief but potent bursts of energy as X-rays.

“Once in a while, these disks will undergo a total
convulsion ““ cause, effectively, an explosion, (and) just
start spewing out X-rays,” Morris said.

Using this method, Muno and his team identified seven sources of
transient X-ray bursts within 75 light years of the supermassive
central black hole, a much higher concentration of black holes than
scientists had imagined to exist.

As the densely packed remnants of dead stars, black holes should
be distributed as evenly throughout the galaxy as other stars are,
Morris said.

But the X-ray bursts, betraying the presence of black holes and
other massive objects, imply that the concentration of black holes
in the central few light years of the galaxy is higher than it
should be, Morris said. According to Morris, this suggests that,
slowly but surely, the black holes are migrating inward.

“We didn’t expect to see black holes (so) close to
the central black hole,” Muno said. “Given the number
of stars … (there was) only a 20 percent chance that one of these
would be so close to the galactic center.”

Muno said this supports Morris’ 1993 prediction that black
holes should congregate at the center of the galaxy, due to a
process by which heavier objects ““ such as black holes
““ sink to the center of the galaxy like rocks tumbling down a
well.

More often than not, Morris said, when a pair of stars encounter
one another, the heavier star gives some of its energy to the
lighter one, flinging its lighter partner into space. The heavier
star sinks slowly toward the supermassive black hole over billions
of years until it, too, meets its fate by merging with the black
hole.

“They’re at the bottom of the well. They can’t
get out ““ it’s inevitable. … There’s no magic
fairy that’s going to pull them out of that pit,”
Morris said.

Collisions of such massive objects as black holes may also cause
ripples in the fabric of space-time, or so-called “gravity
waves,” which ““ if they are detected ““ could be
the first confirmation of a very important prediction of
Einstein’s theory of gravity, Muno said.

Muno said his team plans to continue to use Chandra to study
these “black holes” in the center of the galaxy, to
confirm that the dense objects setting off these X-ray bursts
really are black holes.

“We can’t categorically rule out neutron
stars,” Morris said, explaining that these stars, although
not much more massive than our sun and not nearly as massive as
black holes, could also produce X-ray bursts like those Chandra has
detected. But, he adds, there are ways for scientists to
distinguish between the two, with further research.

“(With Chandra), we’re seeing things that no one had
ever seen before … and confirming things that people did expect
but never thought they could see,” Muno said.