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Distress Signals From Dying Quasars
19 April 1999 6:00 pm
Scientists think they may have heard the whimpers of dying quasars. Using the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA), a team of British and American astronomers has picked up feeble high-energy x-rays from six old, nearby galaxies that they say come from supermassive black holes starving to death. The results were presented at last week's meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, South Carolina.
Astronomers already suspected that the giant elliptical galaxies ASCA observed harbor black holes millions or billions of times more massive than the sun. In five of the six galaxies studied, stars and gas whip around the center at high speeds, apparently in the grip of a powerful gravitational field. But these black holes had seemed quiescent, like the ones thought to sleep at the centers of our own galaxy and others.
Using ASCA, however, Tiziana Di Matteo of the Harvard-Smithsonian Center for Astrophysics and Steven Allen and Andy Fabian of the Institute of Astronomy in Cambridge, U.K., found that a whisper of high-energy x-rays emanates from them--just what you would expect, they say, of a quasar still being fed by a trickle of gas. Instead of forming the flat, dense disk of infalling material thought to surround the black hole in a quasar or active galaxy, the meager infall should form a bloated, tenuous disk, or torus. According to theoretical models, the ionized hydrogen in such a low-density disk would grow very hot, because hydrogen nuclei, or protons, radiate energy slowly. In a denser disk they can transfer energy to electrons--which radiate millions of times more efficiently--but in a rarefied disk, collisions between the protons and electrons would be rare. The superheated gas would slowly leak very high-energy x-rays.
"It's a plausible model," says Bram Achterberg of Utrecht University, "although it's not completely clear that such hot, thick disks can remain dynamically stable over long periods of time." Fabian concedes that he and his colleagues also can't be sure the faint x-rays really are coming from the cores of the elliptical galaxies; ASCA's positional accuracy of half an arc minute is simply not high enough. "There's a lot of galaxy in half an arc minute," he says. But he says that NASA's Chandra X-ray Observatory, slated for launch later this year, will "without doubt deny or confirm our model."