X-ray Data Reveal Black Holes Galore.

Viewed in visible light, the sky appears as a dark expanse, adorned with the twinkling lights of faraway stars. But with an X-ray telescope, the sky seems uniformly bright, bathed in a diffuse glow. For 37 years, astronomers have struggled to find the multitude of pointlike sources that combine to produce this impressionistic glow, known as the X-ray background. Although they have made progress, the limited ability of telescopes to detect X rays in fine detail has hampered their efforts.

Using the orbiting Chandra X-Ray Observatory, a sensitive telescope launched last July (SN: 9/4/99, p. 148), researchers now report that they've pinned down the origin of the background at energies where it had remained most elusive--above 2,000 electronvolts. The results suggest that supermassive black holes lurking at the cores of galaxies are far more common than visible-light observations have revealed.

An intriguing, but much less certain, possibility is that some of the X-ray-bright objects are the signposts of the earliest galaxies to assemble in the universe.

The source of X-ray background is "just the sort of thing one hoped Chandra would discover," comments astronomer Roger D. Blandford of the California Institute of Technology in Pasadena.

The researchers presented their findings this week in Atlanta at a meeting of the American Astronomical Society.

To search for the origins of the X-ray glow, Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md., and Keith A. Arnaud of the University of Maryland, College Park focused on a small patch of sky. Chandra stared at it for 27.7 hours early last December.

"We found enough point sources of the right [intensity] and distribution to account for 80 to 100 percent of the X-ray background," Mushotzky says. By chance, other researchers had extensively studied the same patch of sky in visible light, using one of the twin Keck telescopes--the world's largest optical detectors--atop Hawaii's Mauna Kea.

Comparing the two sets of observations, Lennox L. Cowie and Amy Barger of the University of Hawaii in Honolulu found that about one-third of the X-ray sources reside at the centers of galaxies with brightly shining cores known as active galactic nuclei. Such galaxies are thought to harbor massive black holes, which cause surrounding gas to produce X rays and visible light as the gas gets sucked in.

Another one-third of the X ray-emitting objects lie at the centers of galaxies that radiate very little visible light from their cores. The team suggests that these X rays also come from massive black holes, but ones that are shrouded in dust or unusually quiescent at visible wavelengths.

"The Chandra data show there are at least twice as many" black holes as had been counted in visible light, says Mushotzky.

Another team, led by Gordon Garmire of Pennsylvania State University in State College, has also traced the sources of the high-energy X-ray background and found a multitude of black hole candidates. The group used Chandra to examine a patch of sky that includes the Hubble Deep Field, a region scrutinized in visible light by the Hubble Space Telescope.

Though the number of newly detected massive black holes is not a surprise, these findings highlight their prevalence, Blandford says. Theorists suggest that black holes play a crucial role in galaxy formation.

The remaining X-ray sources found by Mushotzky's team may pose a bigger puzzle. The galaxies they lie within are barely, if at all, detectable in visible light. One explanation, Mushotzky says, is that these galaxies are so young and distant that the vast volume of gas that lies between them and Earth absorbs their light. These X rays could thus represent the "beacons that light the first galaxies," he suggests.

This interpretation is consistent with earlier predictions. Martin J. Rees of the University of Cambridge in England contends, however, "Most astronomers would think it more likely that the sources are [nearer to us] and obscured by dust." The puzzle won't be solved until astronomers manage to measure the distances to the X ray-emitting objects.