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Black hole boldly goes where
no black hole has gone before
EUROPEAN SPACE AGENCY NEWS RELEASE
From
spaceflightnow.com
(Some comments below)Astronomers have found a black hole
where few thought they could ever exist, inside a globular star
cluster. The finding has broad implications for the dynamics of
stars clusters and also for the existence of a still-speculative new
class of black holes called 'intermediate-mass' black holes.
The discovery is reported in the
current issue of Nature. Tom Maccarone of the University of
Southampton in England leads an international team on the finding,
made primarily with the European Space Agency's XMM-Newton
satellite.
Globular clusters are dense bundles of thousands to millions of old
stars, and many scientists have doubted that black holes could
survive in such an exclusive environment. Computer simulations show
that a newly formed black hole would first sink towards the centre
of the cluster but quickly get gravitationally slingshot out
entirely when interacting with the cluster's myriad stars.
The new finding provides the first convincing evidence that some
black hole might not only survive but grow and flourish in globular
clusters. What has astonished astronomers is how quickly the black
hole was found.
"We were preparing for a long, systematic search of thousands of
globular clusters with the hope of finding just one black hole,"
said Maccarone. "But bingo, we found one as soon as we started the
search. It was only the second globular cluster we looked at."
The search continues to find more, Maccarone said, yet only one
black hole was needed to resolve the decades-old discussion about
black holes and globular clusters.
Scientists say there are two main classes of black holes.
Supermassive black holes containing the mass of millions to billions
of suns are found in the core of most galaxies, including our own. A
quasar is one kind of supermassive black hole. Stellar-size black
holes contain the mass of about ten suns. These are created from the
collapsed core of massive stars. Our galaxy likely contains millions
of these black holes.
Black holes are, by definition, invisible. But the region around
them can flare up periodically when the black hole feeds. As gas
falls into a black hole, it will heat to high temperatures and
radiate brightly, particularly in X-rays. Maccarone's team found one
such stellar-mass black hole by chance feeding in a globular cluster
in a galaxy named NGC 4472, about fifty million light-years away in
the Virgo Cluster.
XMM-Newton is extremely sensitive to variable X-ray sources and can
efficiently search across large patches of the sky. The team also
used NASA's Chandra X-ray Observatory, which has superb angular
resolution to pinpoint the X-ray source's location. This allowed
them to match up the position of the X-ray source with optical
images to prove that the black hole was indeed in a globular
cluster.
Globular clusters are some of the oldest structures in the universe,
containing stars over 12 thousand million years old. Black holes in
a cluster would likely have formed many thousand millions of years
ago, which is why astronomers have assumed they would have been
kicked out a long time ago.
Details in the X-ray light detected by XMM-Newton leave little doubt
that this is a black hole - the object is too bright, and varies by
too much to be anything else. In fact, the source is 'extra bright',
- an Ultraluminous X-ray object, or ULX. ULXs are brighter than the
'Eddington limit' for stellar mass black holes, the brightness level
at which the outward force from X-rays is expected balance the
powerful gravitational forces from the black hole. Thus it is often
suggested that the ULXs might be intermediate mass black holes -
black holes of thousands of solar masses, heavier than the
10-solar-mass stellar black holes, and lighter than the million to
thousand million solar mass black holes in quasars. These black
holes might then be the missing links between the black holes formed
in the death throes of massive stars and the ones in the centres of
galaxies.
It is perhaps possible for a stellar-mass black hole to gain enough
mass through merging with other stellar-mass black holes or
accreting star gas to stay locked in a cluster. About 100 solar
masses would do. Once entrenched, the black hole has the opportunity
to merge with other black holes or accrete gas from a local
neighbourhood rife with star-stuff. In this way, they could grow
into IMBHs.
"If a black hole is massive enough, there's a good chance it can
survive the pressures of living in a globular cluster, since it will
be too heavy to be kicked out," said Arunav Kundu of Michigan State
University, a co-author on the Nature report. "That's what is
intriguing about this discovery. We may be seeing how a black hole
can grow considerably, become more entrenched in the cluster, and
then grow some more.
"On the other hand," continued Kundu, "there are a variety of ways
to make ULXs without requiring intermediate mass black holes. In
particular, if the light goes out in a different direction than the
one from which the gas comes in, it doesn't put any force on the
gas. Also, if the light can be 'focused' towards us by reflecting
off the gas in the same way that light from a flashlight bulb
bounces off the little mirror in the flashlight, making the object
appear brighter than it really is."
Ongoing work will help to determine whether this object is a
stellar-mass black hole showing an unusual manner of sucking in gas,
allowing it to be extra bright, or an IMBH. The team, which also
includes Steve Zepf from Michigan State University, and Katherine
Rhode from Wesleyan University, has data for thousands of other
globular clusters, which they are now analyzing in an effort to
determine just how common this phenomenon is.
See original article
here
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From our TPOD "M15:
A Great Globular Cluster":
Within our own galaxy, the
Milky Way, globular clusters or spherical configurations of
stars such as M15 above, are a particularly difficult problem
for a gravity-only cosmology. By what mechanical magic does
gravity hold a million stars together in this way, as a sphere
rather than the familiar disk of gravitational models?
Gravitationally, spherical configurations of stars simply
hanging in space are absurd.
Also see Wal Thornhill's article "The
Madness of Black Holes."
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