Star Cluster Holds Midweight Black Hole, VLA Indicates (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Sun, 24 Jun 2007 17:47:09 -0400
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May 28, 2007
Star Cluster Holds Midweight Black Hole, VLA Indicates
Astronomers using the National Science Foundation's Very Large Array (VLA)
radio telescope have greatly strengthened the case that supermassive black
holes at the cores of galaxies may have formed through mergers of smaller
black holes. Their VLA studies showed that a globular star cluster in the
galaxy M31 probably has a black hole with 20,000 times the mass of the Sun
at its core.
"That amount of mass is midway between the black holes left when giant stars
explode as supernovae and the supermassive black holes with millions of
times the mass of the Sun. It suggests that there is a clear path for
forming the supermassive ones through successive mergers of smaller black
holes," said James Ulvestad, of the National Radio Astronomy Observatory.
Ulvestad, Jenny Greene of Princeton University, and Luis Ho of the
Observatories of the Carnegie Institute of Washington presented their
findings to the American Astronomical Society's meeting in Honolulu, Hawaii.
Black holes appear to be intimately connected with the formation of massive
spherical bulges in galaxies. Astronomers have found a direct relationship
between the mass of the black hole in such a galaxy and the mass of its
central bulge. However, it is unclear whether small galaxies contain smaller
black holes, and their discovery may lead to new insights about the impact
of black holes on galaxy formation. As Greene stated, "In recent years, we
have been detecting black holes with masses between 100,000 and a few
million times the mass of the Sun, but less massive objects have been
exceptionally difficult to find."
Based on observations with optical telescopes, Karl Gebhardt of the
University of Texas at Austin, R. Michael Rich of UCLA, and Ho, suggested in
2002 that the globular cluster G1 in the Andromeda Galaxy (M31) contains a
compact concentration of mass that is intermediate in mass between stellar
and supermassive black holes. Other researchers disputed that conclusion.
According to Ho, "In 2005, we obtained better data that clinched the case
that the cluster really does contain a dark object with 20,000 times the
mass of the Sun. What we can't be sure of, however, is whether the dark mass
is a single object -- that is, an intermediate-mass black hole -- or a
cluster of smaller dark objects such as neutron stars or stellar-sized black
holes."
"Since this globular cluster, G1, is by far the best candidate for
containing such an intermediate-mass black hole, we felt it was important to
help resolve the question," Ho added.
Last year, researchers detected X-rays emitted from G1. That allowed
Ulvestad and his team to apply a test that could distinguish between an
intermediate-mass black hole and the smaller compact objects. Astronomers
have found that, for a given X-ray brightness, a supermassive black hole is
much brighter at radio wavelengths than a stellar-mass black hole.
Using the VLA, Ulvestad, Greene and Ho found that the radio brightness of G1
was between what would be expected for a stellar-mass black hole and what
would be expected for a supermassive one. "The radio brightness nicely fits
the prediction for a 20,000-solar-mass black hole," Ulvestad said.
Some globular clusters in our own Milky Way galaxy are suspected to contain
black holes with masses just a few hundred times the mass of the Sun. These
may be detected when the Expanded VLA, with much greater sensitivity than
the current telescope, comes on line soon after 2010. "With this capability,
we could close the gap between black holes with masses 10 times that of the
Sun and those with masses more than 10,000 times that of the Sun," Ulvestad
said. "This should lead to a greater understanding of their importance as
stepping stones toward the most massive black holes we observe in the
centers of many galaxies."
The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
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