In 2004 Stephen Hawking famously changed his mind about black holes – a place in space where gravity pulls so much that not even light can escape from it. Now astronomers have made a new discovery that may well once again change the way science sees black holes, or more precisely the company they keep. A team of international astronomers have discovered two black holes, bucking theorists who suggested that there could only be one. The discovery is making scientists rethink their understanding of the environment in globular star clusters, tight-knit collections containing hundreds of thousands of stars.
Using the Very Large Array (VLA) based in New Mexico, United States the team of astronomers studied a globular cluster called Messier 22 (M22), a group of stars more than 10 000 light years from Earth. The astronomers were scanning the group hoping to find evidence of a rare type of black hole in the cluster's centre called an intermediate-mass black hole.
However, their search revealed something unexpected. Instead of finding a black hole more massive than those larger than the Sun's mass, but smaller than the supermassive black holes found at the cores of galaxies, they found two smaller black holes. What was surprising about this is that most theorists believe that there should be at most one black hole in the cluster.
Dr Tom Maccarone, a reader in Astronomy at the University of Southampton, United Kingdom and who developed the methodology for the study, spoke about the discovery. 'I had actually suggested several years ago that there were probably black holes lurking among the X-ray sources that had already been seen in globular clusters, and that the one way to pick the black holes sucking gas in, apart from other types of faint X-ray sources, would be to look for the radio emission, but I didn't expect that this particular cluster would be the best place to look. It was still incredibly exciting to see this result and I'm optimistic that we will find more of these objects in other clusters in the future.'
Black holes are left over after very massive stars have exploded as supernovae, and in a globular cluster many of these stellar-mass black holes probably were produced early in the cluster's 12-billion–year history as massive stars rapidly passed through their life-cycles. Simulations indicated that these black holes would fall toward the centre of the cluster and then begin a violent gravitational dance with each other, in which all of them, or perhaps all but a single one, would be thrown completely out of the cluster.
'There is supposed to be only one survivor possible,' notes Jay Strader of Michigan State University and the Harvard-Smithsonian Center for Astrophysics in the United States. 'Finding two black holes, instead of one, in this globular cluster definitely changes the picture,' he says.
The astronomers have already come up with some possible explanations. One possibility is that 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 two black holes discovered with the VLA were the first stellar-mass black holes to be found in any globular cluster in our own Milky Way galaxy, and also are the first found by radio, instead of X-ray, observations. Future VLA observations will help us learn about the ultimate fate of black holes in globular clusters.