Galactic and stellar black holes get real

"In my entire scientific life . . . the most shattering experience has been the realization that an exact solution of Einstein's equations of general relativity . . . provides the absolutely exact representation of untold numbers of massive black holes in the universe."

 In 1975, when the late Nobel laureate Subrahmanyan Chandrasekhar wrote those words, the idea of black holes required a stretch of the imagination. Today, few astronomers doubt the existence of these invisible bodies, which exert the most extreme gravitational tug in the cosmos.

 Moreover, at a meeting of the American Astronomical Society in Toronto this week, researchers announced that three galaxies, all within 50 million light-years of Earth, have central black holes ranging from 50 million to 500 million times the mass of the sun. Douglas O. Richstone of the University of Michigan in Ann Arbor and his colleagues also said that data, some of it previously reported, bring to 21 the number of nearby galaxies that harbor black holes.

 This evidence bolsters the argument that nearly every galaxy contains a black hole. Although Richstone said he would not yet stake his life on that assertion, "I'd bet my car on it, and it's a pretty good car." Researchers conclude that only the gravitational tug of a highly condensed central mass can account for the large velocities of stars at the centers of these galaxies.

 The census of black holes is expected to grow dramatically after the Hubble Space Telescope gets a new spectrograph next month and begins observing the motions of stars in dimmer galaxies. However, the data already suggest a trend: A galactic black hole's mass is proportional to the galaxy's mass, notes Richstone. Martin J. Rees of the University of Cambridge in England proposes that black holes formed simultaneously with the central bulge of galaxies as gas migrated to the core.

 Another team, studying lower-mass black holes, reported the first evidence of an event horizon, one of the weirdest properties of these unseen bodies. Ramesh Narayan and his colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., examined a group of objects, called X-ray novas, that consist of a visible star locked in the gravitational embrace of an unseen companion. The X rays come from hot gas torn from the visible star and falling onto a disk surrounding the companion.

 That companion is sometimes a black hole and sometimes a neutron star, the collapsed remains of a dead star. Two years ago, Narayan and a colleague theorized that under special circumstances, the pattern of X-ray emission could reveal the companion's identity.

 When gas from the visible star transfers onto the disk extremely slowly, it grows as hot as 1 trillion kelvins but radiates only weakly. Gradually, the hot material spirals inward. If the companion is a neutron star, the material will radiate its energy as X rays when it hits the star's surface. If the companion is a black hole, however, the energy vanishes from sight, swallowed inside the event horizon -- the envelope thought to surround a black hole and from which not even radiation can escape.

 Narayan's team used the Japanese X-ray satellite ASCA to observe V404 Cygni, an X-ray nova known to contain a black hole. The radiation pattern matched that predicted for an event horizon. "The evidence from our study says that these objects really do have event horizons," says Narayan.