Surprise black hole duo discovery

An international team of astronomers have made an unexpected discovery about globular star clusters – tight-knit collections containing hundreds and thousands of stars. The astronomers were studying globular cluster called Messier 22 and were hoping to find evidence of a rare type of black hole in the cluster’s centre called an intermediate-mass black hole.  Suprisingly however, they found two smaller black holes instead.

Dr Tom Maccarone from the University of Southampton who developed the methodology for the study said: "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."

The team used the National Science Foundation’s Karl G. Janksky Very Large Array (VLA) in New Mexico to study Messier 22, which is more than 10 000 light-years from Earth. The findings are published in Nature. Simulations have suggested that black holes in a globular cluster would fall towards the centre of a cluster and then begin a violent gravitational dance with each other, in which all of them or perhaps all but one would be thrown out of the cluster completely.

“There is supposed to be only one survivor possible. Finding two black holes, instead of one, in this globular cluster definitely changes the picture.” said Jay Strader of Michigan State University.

The two black holes discovered were the first stellar-mass black holes to be found in any globular cluster in the Milky Way. Future VLA observations will enable astronomers to learn about the ultimate fate of black holes in globular clusters.