Two black holes survive in star cluster

MICHIGAN STATE (US) — Scientists are re-thinking the fates of black holes after discovering two together in the same star cluster.

The discovery was surprising, says Jay Strader, assistant professor of physics and astronomy at Michigan State University. Normally when black holes live in such an environment, it is assumed that only one will survive, explains Strader.

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.

Images of the globular star cluster M22. On the left is an optical image, showing the dense stellar environment where the two newly discovered black holes dwell (orange square). The right panel is a detail of the VLA radio image. The white point sources are the radio detections of the black holes. (Credit: D. Matthews/A. Block/NOAO/AURA/NSF)


“There is supposed to be only one survivor possible,” Strader says. “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.

The research by Strader 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. The team used images of unprecedented depth observed at radio wavelengths and reported findings in the journal Nature.

“Future radio observations with the Very Large Array telescope will help us learn about the ultimate fate of black holes in globular clusters,” says Laura Chomiuk, a postdoctoral 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 UK; James Miller-Jones, of the International Centre for Radio Astronomy Research at Curtin University; and Anil Seth of the University of Utah.

Source: Michigan State University