Jan 4, 2007
Telescopes find black holes by looking for bright X-ray sources
Astronomers say the discovery is interesting because many doubted black holes could exist in such locations.
Some computer simulations had suggested a newly formed black hole would simply be ejected from the cluster as a result of gravitational interactions.
Tom Maccarone, of the University of Southampton (UK), and colleagues report the finding in the journal Nature.
It was made using the European Space Agency's XMM-Newton satellite, with follow-up observations on the US space agency's Chandra Space Telescope - both are sensitive to the X-ray light that is emitted when gas consumed by a black hole is torn apart.
The international team says its work provides the first convincing evidence that some black holes might not only survive but grow and flourish in globular clusters.
What has astonished the scientists 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 Dr Maccarone. "But bingo, we found one as soon as we started the search. It was only the second globular cluster we looked at."
The black hole is located in a globular cluster associated with a galaxy named NGC 4472, some 55 million light-years away.
Globular clusters are among the oldest structures in the Universe. They contain thousands to millions of stars packed into a region of space just a few tens of light-years across.
These high densities should lead to frequent interactions and even collisions; and some models have suggested that large black holes - several hundred times the mass of our Sun - could develop in the densest inner regions of clusters.
Other simulations, however, predict that such gravitational interplay would probably eject most or all of the black holes that form in such an environment.
The team is uncertain about the size of the NGC 4472 hole; but if it is reasonably large - and one interpretation of the X-ray data suggests it could be 400 times the mass of our Sun - it might have been able to anchor itself in the cluster, said co-author Arunav Kundu of Michigan State University, US.
"This is one of the interesting aspects of this study," he told BBC News.
"People have seen stellar-sized black holes that form from [an exploded] star, and then there are the super-massive black holes at the centres of galaxies that are millions of times the mass of our Sun - but there's nothing in between.
"Some people expect that globular clusters might be the environment where you would see intermediate-mass black holes."