Original URL: http://www.theregister.co.uk/2012/10/04/messier_22_double_black_hole/

Boffins get black hole double-vision

Messier 22 springs astro-physics surprise

By Richard Chirgwin

Posted in Science, 4th October 2012 01:45 GMT

The Messier 22 globular star cluster has yielded a messier-than-expected observation: instead of the black hole astronomers expected to find in its centre, there are two.

Conducted by the University of Southampton’s Dr Tom Maccarone, Michigan State University assistant professor Jay Strader and others, and using the NSF’s Karl G Jansky Very Large Array in New Mexico, the researchers analysed observations of Messier 22 hoping to find an intermediate-mass black hole.

Instead, they found two smaller black holes in a pas de deux some distance away from the cluster’s centre. They estimate that each of the objects is about ten-to-twenty times the mass of the Sun.

Part of the Milky Way, Messier 22 is about 10,000 light-years from Earth.

"We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the centre," study co-author James Miller-Jones from the International Centre for Radio Astronomy Research says.

The find is surprising because mathematical simulations predict that of the black holes that might have once formed in a globular star cluster, only one should survive – not because it will absorb the others, but because in their “gravitational dance”, the others should have been thrown out of the cluster.

In this release, via AlphaGalileo, the researchers propose a couple of possible mechanisms to keep two black holes in the cluster.

“First, the black holes themselves may gradually work to puff up the central parts of the cluster, reducing the density and thus the rate at which black holes eject each other through their gravitational dance. Alternatively, the cluster may not be as far along in the process of contracting as previously thought, again reducing the density of the core”, the announcement states.

The discovery marks a couple of other firsts as well: it’s the first time black holes have been observed in a globular star cluster within the Milky Way; and they’re the first black holes discovered by radio rather than X-ray observations. ®