Quasars, objects larger than stars and found only in the centers of galaxies, are the brightest celestial objects in the universe. They are powered by massive black holes -­ objects so dense that even light cannot escape their gravity. As stars and interstellar gas fall into the black holes, they swirl around them, and then are swallowed up ­ but not before giving off bright light at nearly all wavelengths of the electromagnetic spectrum.

University of Arizona astronomer Jill Bechtold and her colleagues used NASA's Chandra X-ray Observatory in studying a large sample of quasars no closer than 12 billion light years from Earth.

Their large survey includes three newly discovered quasars 13 billion light years from Earth -- the earliest, most distant quasars yet found. They existed when the universe was only about one billion years old, or about 7 percent of its present age.

"The objects which are shining as quasars today have lower mass black holes than the objects we see shining as these very luminous quasars in the distant past," Bechtold said. "The ancient quasars are very bright and also very rare. Basically, their black holes must exist today, but they just aren't 'quasing.' We don't know why. Probably their 'fuel' -­ the stars and gas falling into the black hole ­- has become scarce."

The three youngest quasars were discovered at optical wavelengths by the Sloan Digital Sky Survey last year. Bechtold and others have been studying X-ray observations of the three quasars made Jan. 29, 2002, with Chandra.

Bechtold and her group proposed their survey of ancient quasars in early 1998. They observed 14 very distant quasars in the early universe, between 12 billion and 12.5 billion light years away, after the launch of Chandra in mid 1999.

Bechtold's results will be published in the Astrophysical Journal.

Astronomers have suspected that the most distant quasars have more massive black holes than do nearer ones, Bechtold said. Theory predicts that coronal hot gas swirling in the accretion disk around a central supermassive black hole will emit less X-ray energy than will coronal hot gas in the disk around a smaller, closer black hole, she said.

"Our X-ray data are consistent with that," she said.

The results contradict theory that says black holes and galaxies become more massive through gravitational mergers as the universe evolves, Bechtold said.

"We see that there is strong evolution in the population of quasars. Relatively few are shining as brilliantly as the quasars we observed with Chandra, the quasars of more than 10 billion years ago.

"We want to understand what drives this evolution, and the X-rays can help tell us about what is happening close to the 'central engine' or black hole in the centers of quasars," Bechtold said.

Other astronomers reporting papers based on Chandra observations of the three youngest quasars include Niel Brandt of Pennsylvania State University, Smita Mathur of Ohio State University and Daniel Schwartz of the Harvard-Smithsonian Center for Astrophysics.

They do not reach the same conclusions, but do agree that black holes producing the X-rays are huge, given their relative youth. By various estimates, the three quasars each are somewhere between one billion and 10 billion times as massive as our sun, Bechtold said.

By comparison, the black hole at the center of the Milky Way is believed to be only about 3 million times as massive as the sun.


[Contact: Jill Bechtold, Lori Stiles]

01-Apr-2002