Black holes seem to come in only two varieties: "supermassive" ones, which power quasars and weigh millions to billions of times more than the sun, and "stellar mass" black holes, which have about the mass of one large star. But in this month's Astrophysical Journal and Astrophysical Journal Letters, astronomers report the discovery of a new class of black holes right in the middle.
A black hole is an object so dense that even light cannot escape its gravitational pull. Astronomers believe that stellar mass black holes form when a massive star reaches the end of its life and collapses to a point of infinite density. Supermassive black holes are more mysterious. "No one really knows" their origin, says astronomer Richard Griffiths of Carnegie Mellon University in Pittsburgh. One theory holds that they form in "starburst galaxies," which contain seething cauldrons of young, hot stars that flare up suddenly in the galaxy's core and burn out just as fast, leaving behind a pile of stellar debris, including stellar mass black holes. These may lump together and feed off the remains of other stars, thus growing into giant black holes.
To test this hypothesis, many researchers have searched starburst galaxies for the intermediate-size black holes that should form along the way. Black holes are invisible, of course, but the x-rays emitted by hot gas gathering nearby can reveal their presence and give clues about their mass. The searches have turned up a menagerie of potential midsize black holes, but no confirmed cases.
Now, two groups have taken another look at several x-ray sources. Griffiths and his colleague Andrew Ptak pointed the ASCA satellite at one source in galaxy M82; x-ray astronomers Richard Mushotzky and Ed Colbert at NASA's Goddard Space Flight Center in Greenbelt, Maryland, surveyed the same source plus 38 others with ROSAT, another satellite. By comparing the luminosity and spectrum of the x-rays from the sources to those of known black holes at both ends of the mass range, the two groups conclude that about a dozen of the previous candidates are indeed middleweight black holes. They probably weigh between 100 and 10,000 times the mass of the sun, says Griffiths, who says the studies are "a major clue" that the objects are supermassive black holes in their infancy.
Astrophysicist Jean-Pierre Lasota of the Observatoire de Meudon in France agrees. "They have done a very uncertain exercise very carefully," he says.