Astronomers discover twin black holes in globular cluster

Thursday, October 04, 2012
Artist's conception of black hole in globular cluster
CREDIT: Benjamin de Bivort; Strader, et al.; NRAO/AUI/NSF
Charlottesville, Virginia: In an unexpected finding, astronomers recently discovered two black holes while searching for a unique black hole in a tight cluster of stars 10,000 light years away from Earth. Instead of finding one black hole, scientists with the National Radio Astronomy Observatory (NRAO) found two – twins – something that surprised them because, according to modern theory, there should only be one black hole in a cluster.

The discovery could make scientists reconsider their current understanding of the environment in globular star clusters.

The astronomers made the find while observing Messier 22 (M22), a globular star cluster containing hundreds of thousands of stars, with the Very Large Array (VLA) radio telescope located in New Mexico.  They were searching for proof of a rare black hole known as an intermediate-mass black hole.

Unlike the supermassive black holes  found at the center of galaxies, an intermediate-mass black hole has 10- to several tens of times more mass than the Sun, but is comparatively smaller in size.

“We didn’t find what we were looking for, but instead found something very surprising – two smaller black holes,” said Laura Chomiuk of Michigan State University and the National Radio Astronomy Observatory.

Black holes are concentrations of mass  so dense that nothing, not even light, can escape them.  The NRAO astronomers think most black holes found within the globular cluster were likely produced early in the cluster’s 12-billion-year history after massive stars exploded as supernovae.

Past scientific simulations suggest black holes fall toward the center of the cluster, beginning a violent gravitational competition with each other.  In the end, only a single black hole remains in the cluster.

“There is supposed to be only one survivor possible,” said Jay Strader of Michigan State University and the Harvard-Smithsonian Center for Astrophysics. “Finding two black holes, instead of one in this globular cluster, definitely changes the picture.”One possible explanation is that the black holes may still be working to enlarge the center of the star cluster, reducing its density and slowing down the rate at which the black holes eject each other as they compete to become the cluster’s lone entity.

On the other hand, the astronomers think the reduction of the black hole’s density and ejection rate could also be due to the fact that the star cluster is not as far along in its process of contracting as previously thought.

“Future VLA observations will help us learn about the ultimate fate of black holes in globular clusters,” Chomiuk said.

paper on this finding was just published in the journal “Nature”.

According the NARO, these twin black holes are also the first stellar-mass black holes to be found in any globular cluster in our Milky Way Galaxy. A stellar-mass black hole is formed by the collapse of a star that has a mass that is generally 10 to 24 times that of the Sun, as compared to the giant “supermassive” black holes, which are millions, if not billions, of times as massive as the Sun.

Astronomers believe supermassive black holes lie at the center of virtually all large galaxies, including ours.
Next Post »