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Twin Black Holes

By Stephen Luntz

Theories about the evolution of globular clusters are being revised with the unexpected discovery of two black holes close to the centre of M22, a bright cluster located near the galactic bulge in Sagittarius.

“The study was originally searching for just one larger black hole within the cluster of hundreds of thousands of stars which, when viewed with the naked eye, resembles a hazy ‘puff’ of light,” said Dr James Miller-Jones of Curtin University and the International Centre for Radio Astronomy Research.

What they found was much more exciting, and has been published in Nature. “Simulations of how globular clusters evolve show many black holes are created early in a cluster’s history,” Miller-Jones continues. “These then sink towards the middle of the cluster, where they begin a chaotic dance leading to most being thrown out of the cluster until only one surviving black hole remains.”

The discovery of the two holes, each lying within a few light years of the cluster’s centre, challenges these beliefs. The finding is particularly surprising because no black hole has been observed previously in one of our own galaxy’s globular clusters, although a handful have been found in clusters around other galaxies.

Miller-Jones explains that black holes are hard to spot when they are not feeding rapidly on a companion star, and that in-depth searches have not been done specifically seeking black holes in most of the Milky Way’s globular clusters. “We were looking to see if there might be an intermediate mass black hole here. We know that as black holes get larger they get easier to spot in the radio spectrum.”

The discovery was made with the recently upgraded Very Large Array in New Mexico. It is the first time a black hole’s signature has been found first in the radio spectrum rather than being discovered with an X-ray telescope and confirmed via radio waves.

While accurate mass measurements have been difficult, Miller-Jones says two techniques have produced estimates of 10–20 solar masses, and both definitely qualify as small, rather than intermediate, black holes. Both are feeding quite slowly, making them relatively hard to detect, but Miller-Jones says the team has ruled out all alternative explanations it can think of for the observed signal.

The age of most of M22’s stars indicate that black holes should have formed close to 12 billion years ago, but Miller-Jones says: “There is some evidence of a spread of ages, indicating either a second round of star formation or that M22 was formed from the merger of two globular clusters”.

In the latter case it is possible these are the black holes from the centre of each cluster and have not yet had time for one to expel the other. An even more exotic theory holds that the interaction of both holes and the nearby stars is heating the centre of the cluster to an extent that slows the core’s collapse and delays one hole’s expulsion.