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Black hole boldly goes where no black hole has gone before (Forwarded)



 
 
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Old January 4th 07, 09:49 PM posted to sci.astro
Andrew Yee
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Default Black hole boldly goes where no black hole has gone before (Forwarded)

ESA News
http://www.esa.int

3 January 2007

Black hole boldly goes where no black hole has gone before

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.

Note to editors

The findings appear online in the 4 January issue of the journal Nature,
in the article titled: "A black hole in a globular cluster", by Thomas J.
Maccarone, Arunav Kundu, Stephen E. Zepf and Katherine L. Rhode.

For more information:

Tom Maccarone, University of Southampton, UK
Email: tjm @ phys.soton.ac.uk

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.schartel @ sciops.esa.int

More about ...

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

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[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEML0QZTIVE_index_1.html ]

 




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