Star cluster supports pair of black holes

Research by Michigan State University astronomers has scientists re-thinking the fates of black holes, particularly in groups of stars known as globular clusters.

The research of Jay Strader, MSU assistant professor of physics and astronomy, and colleagues focused on a cluster called Messier 22, or M22, a collection of hundreds of thousands of stars located about 10,000 light years from Earth. Using images of unprecedented depth observed at radio wavelengths, Strader and his team were surprised to find not one but two black holes in the cluster.

It was surprising, Strader said, because normally when black holes live in such an environment, it is assumed that only one will survive. The theory is that when black holes form in a star cluster, they tend to fall toward the center of the cluster. It’s then that they start a violent gravitational dance in which all of them, except one, are unceremoniously tossed from the cluster.

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

“The fact that we discovered two in this cluster suggests that theory isn’t right. These clusters are hanging onto more black holes, but the reason is not clear.”

One theory is that the black holes themselves gradually expand 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.

“Future radio observations with the Very Large Array telescope will help us learn about the ultimate fate of black holes in globular clusters,” said Laura Chomiuk, an MSU post-doctoral fellow and member of the research team.

The two black holes were discovered using the National Science Foundation’s Karl G. Jansky Very Large Array telescope in New Mexico. They were the first 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.

Strader and Chomiuk worked with Thomas Maccarone of the University of Southampton in the U.K.; James Miller-Jones, of the International Centre for Radio Astronomy Research at Curtin University; and Anil Seth of the University of Utah.

The scientists published their findings in the Oct. 3 issue of the journal Nature.

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1 thought on “Star cluster supports pair of black holes”

  1. Surely all this means is that one of the Black Holes hasn’t been ejected…yet. Perhaps we’re simply seeing a work in progress.

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