PASADENA, CALIFORNIA--Like the glint of diamonds in the dust, brilliant pinpricks of x-rays have led three independent teams to a precious discovery: Nearly 100 surprisingly heavy black holes in nearby galaxies. A handful of similar objects had been seen before, but the teams never expected to find so many more. They suggest these so-called intermediate mass black holes could coalesce to form the supermassive black holes thought to inhabit nearly every galaxy.
Black holes once seemed to come in two drastically different varieties. Those in the Milky Way and nearby galaxies had about the same mass as the sun; the rest were at least a million times heavier and lived in galaxies at the distant fringes of the universe. In between, there was nothing. The gaps began to fill in 2 years ago. First, astronomers using the Hubble Space Telescope discovered that the supermassive black holes were common in nearby galaxies too, and every galaxy could harbor one of the dark beasts. Then, the ASCA x-ray satellite spotted an unusually bright x-ray source in the galaxy M82, thought to come from the superheated gas surrounding a black hole weighing about 100 solar masses. But was it a rarity?
Absolutely not, says a chorus of speakers here at this week's meeting of the American Astronomical Society. Using images of 40 nearby galaxies taken with the Chandra X-ray Observatory, three teams have brought the count to almost 100. Goddard Space Flight Center astrophysicist Kim Weaver, who led one of the teams, argues that these black holes could spiral to the center of the galaxy and coalesce into a supermassive hole. "There is definitely enough material there to do this," agrees Andrew Ptak, an astrophysicist at Carnegie Mellon University in Pittsburgh, who leads a second team.
Not everyone is convinced that the middleweight holes can add up. "You need a lot of hundred-solar-mass black holes to make a million-solar-mass black hole," says Nick Scoville, an astronomer at the California Institute of Technology. But then, Chandra has only just begun to hunt.