The astronomers studied a globular cluster called Messier 22 more than 10,000 light-years from Earth, hoping to find evidence of a rare type of black hole in the cluster's center, what scientists call an intermediate-mass black hole. Such black holes are more massive than some --a few or more times the sun's mass -- but smaller than the super-massive black holes found at the cores of galaxies.
"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. "That's surprising because most theorists said there should be at most one black hole in the cluster."
Theory up to now holds multiple black holes should not exist in a globular cluster that is 12 billion years old.
"Simulations of how globular clusters evolve show many black holes are created early in a cluster's history," said James Miller-Jones of Australia's International Center for Radio Astronomy Research, which collaborated with the U.S. astronomers in the research.
"The many black holes 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.
"We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the center, which means all the theory and simulations need refinement."
The great age of the cluster means the discovery is a puzzle since the "winnowing" of multiple black holes should have ended long ago, researchers said.
"There is supposed to be only one survivor possible," said Jay Strader of Michigan State University and the Harvard-Smithsonian Center for Astrophysics.