October 3, 2012
Black Holes Change Perspective Of Globular Star Clusters
Lee Rannals for redOrbit.com - Your Universe Online
Astronomers used VLA to study the globular cluster known as Messier 22, which is a tight-knit collection of hundreds of thousands of stars that lie 10,000 light-years away from Earth.
The team had hoped to find evidence for a rare type of black hole in the cluster's center, and wanted to find what scientists call an intermediate-mass black hole. This type of black hole is more massive than those a few more times the Sun's mass, but are smaller than the supermassive black holes.
"We didn´t find what we were looking for, but instead found something very surprising -- two smaller black holes," Laura Chomiuk, of Michigan State University and the National Radio Astronomy Observatory, said in a prepared statement. "That´s surprising because most theorists said there should be at most one black hole in the cluster."
Black holes are left over after massive stars explode into supernovae. Many of these stellar-mass black holes were most likely produced early in the cluster's 12-billion-year history as massive stars passed through their life cycles.
Simulations done by scientists indicate that these black holes would fall toward the center of the cluster, then begin a violent gravitational dance with each other. During this dance, all but a single one would be thrown completely out of 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."
The astronomers believe the black holes may gradually work to puff up the central parts of the cluster, which reduce the density and thus the rate at which black holes eject each other through their gravitational dance.
Also, they said the cluster may not be as far along in the process of contracting as previously thought, which would reduce the density of the core as well.
"Future VLA observations will help us learn about the ultimate fate of black holes in globular clusters," Chomiuk said.
The two black holes discovered with the VLA were the first stellar-mass black holes to be found in any globular cluster in our Milky Way Galaxy. They are the first found by radio observations, rather than X-ray observations.
The team worked with Thomas Maccarone of the University of Southampton in the U.K., James Miller-Jones, of the International Center for Radio Astronomy Research at Curtin University in Australia, and Anil Seth of the University of Utah.
The astronomers will be reporting their findings in the journal Nature on Thursday.