Boldly Going Where No Black Hole
Has Gone before
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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 star 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.
Global clusters are dense bundles of thousands to millions of old stars, and
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 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 phenomena is. just
how common this phenomenon is.
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