Around the cosmos, black holes aren't known for being the nicest neighbors. They tend to make their presence felt in unpleasant ways, pulling nearby matter inward even as they belch out violent blasts of plasma and radiation. Put two of them in the same neighborhood and the resulting tug-of-war can quickly turn ugly. But in a cluster of stars within the Milky Way, two black holes appear to have taken up residence in surprisingly close proximity.

The two relatively lightweight black holes in M22, a so-called globular cluster some 10,000 light-years away containing hundreds of thousands of stars, may represent a much larger total population, which would run counter to predictions that gravitational interactions between black holes in the cluster would eject almost all the black holes in short order. Astronomers from the U.S., England and Australia announced their discovery in a study published in the October 4 issue of Nature. (Scientific American is part of Nature Publishing Group.)

The researchers used the recently upgraded Very Large Array (VLA), a network of radio dishes near Socorro, N.M., to look for an intermediate-mass black hole at the center of M22. These elusive, middleweight objects weigh in at thousands of times the mass of the sun—as compared with the supermassive black holes at the centers of galaxies, which contain millions or even billions of solar masses. What the astronomers found instead was a pair of even lighter black holes that form from the collapse of massive stars. Each of the so-called stellar-mass black holes in M22 carries 10 to 20 times the mass of the sun.

"It's sort of surprising, because the theories that have been made had sort of concluded that there ought to be few or no black holes in these globular clusters," says lead study author Jay Strader, an astronomer at Michigan State University. "We know that black holes get made in globular clusters—black holes get made everywhere that there are massive stars. The question is, What happens to them? Do they stay or do they get kicked out?"

Past studies had favored the "kicked out" hypothesis, but the new finding by Strader and his colleagues suggest that the ejection process may not be as efficient as had been assumed.

"They criticize the theory a little bit, and they should," says Simon Portegies Zwart, an astrophysicist at Leiden University in the Netherlands who did not contribute to the new research. "Once in a while the observers surprise the theorists, and once in a while the theorists surprise the observers."

Strader and his colleagues concluded that the two newfound objects, M22-VLA1 and M22-VLA2, are probable black holes because they appear in VLA radio images of M22 but not in archival x-ray data of the same cluster. That emission pattern all but rules out other types of compact objects common to a globular cluster—M22-VLA1 and M22-VLA2 emit too many radio waves to be white dwarfs (dense remnants of spent stars), and not enough x-ray emission to be neutron stars (even denser remnants of collapsed massive stars).

The fact that the two objects are aglow in radio waves at all requires a special set of circumstances—that each of the black holes is currently feeding on its own close-orbiting stellar companion, most likely a white dwarf. Those unusual conditions make a black hole in M22 a bit like a cockroach in the kitchen—for every one you see, there may be several lurking just out of sight. "For us to see them in the radio, they have to not just be in binaries, but they have to be in binaries that are close enough that mass transfer is actually taking place," Strader says. He and his colleagues estimate that there could be as many as 100 low-mass black holes in the globular cluster. "I think it's pretty unlikely that these objects are the only black holes in M22," he notes.

If black holes can indeed coexist more peacefully than had been presumed, then it falls to theoretical astrophysicists "to scratch their heads and see what they can make out of it," Portegies Zwart says. "It is time to crank the computers up again and see if we can understand this result from first principles. And that will be a major endeavor."