WASHINGTON, Sept. 17 (UPI) -- For the first time, astronomers have found medium-sized black holes among some very old star clusters, a discovery that adds new evidence to help solve an abiding cosmic conundrum: Which came first, galaxies or black holes?
The black holes were discovered in two globular clusters, M15 in the constellation Pegasus within the Milky Way and G1 in the neighboring Andromeda Galaxy, Roeland Van Der Marel, an astronomer at the Space Telescope Science Institute in Baltimore, said at a NASA news conference on Tuesday.
The findings, made by the Hubble Space Telescope, are especially intriguing, said Karl Gebhardt of the University of Texas at Austin. It turns out the masses of the newly discovered black holes, relative to their host star clusters, correlate exactly with the masses of the largest black holes relative to their host galaxies, he explained.
"When you talk about how tight the correlation is, that's where it gets really amazing," said Gebhardt. "It's essentially perfect. Almost never do we have a situation where nature is perfect."
Black holes are celestial objects packed so densely with matter that not even light can escape their gravitational pull. They are found by measuring the gravitational effects on the motions of nearby stars and objects.
Before Tuesday's announcement, black holes had been found only in two extreme sizes: supermassives that are millions to billions times the mass of our sun and found in the centers of galaxies, and stellar-mass black holes that are the compressed remains of supernova explosions. The latter have been found circling stars as well as free-floating in space.
Other theories suggest there may be microscopic-sized black holes traveling through space, but so far there has been little evidence of their existence.
The black hole in M15, located about 32,000 light-years from Earth, has a mass about 4,000 times larger than the sun's. The G1 cluster, about 2.2 million light-years away, has a black hole estimated to be 20,000 solar masses, said Van Der Marel.
The size of the black holes are about 0.5 percent of the mass of the clusters -- the same ratio between galactic-center supermassive black holes and their host galaxies.
The twin findings are expected to revive debate about how common black holes are and how they relate to galaxy star cluster formation.
Globular clusters are among the oldest entities in the universe and astronomers suspect if M15 and G1 are representative, the clusters most likely had black holes when they formed.
An alternative theory of black hole formation is black holes are created by matter condensing at the centers of galaxies.
"I believe that galaxy formation was triggered by black holes," said Marcia Rieke, with the Steward Observatory at the University of Arizona, Tucson.
If that suspicion is correct, she added, then the correlation between the mass of a black hole and its host galaxy or star cluster may change as even more ancient objects are imaged, a prime science goal of the next-generation space telescope -- recently named the James Webb Space Telescope.
"The intermediate-mass black holes ... may be the building blocks of the supermassive black holes that dwell in the centers of most galaxies," said Gebhardt.
(Reported by Irene Brown, UPI Science News, at Cape Canaveral, Fla.)
