The universe now appears to contain far more black holes than previously known, from ancient monsters lurking in galaxies at the edge of space-time to tiny "naked" holes drifting invisibly through the void. And the exotic objects may have played a crucial role in shaping the visible cosmos.
Those are the conclusions of new research released today from several separate groups of astronomers studying the sky with half a dozen telescopes.
The findings, announced at a meeting of the American Astronomical Society, indicate that "supermassive" black holes--those with masses millions of times greater than the sun's--are an important, if not essential, part of galaxy formation. In many cases, some astronomers now think, giant black holes formed even before the stars that surround and feed them.
Thus the new observations help to answer "what is the fundamental question in modern astrophysics: How did matter get to be as lumpy as it is," said Virginia Trimble of the University of California at Irvine, who did not participate in the research.
Black holes form when enormous amounts of matter--anywhere from about three times to billions of times as massive as the sun--collapse to a point. The resulting gravitational field is so strong, and warps space so violently, that it swallows everything within its perimeter, or "event horizon." Even light cannot escape.
There are, however, a few ways of detecting them. One occurs when neighboring gas and stellar matter swirls into a black hole, forming an "accretion disk" in which the gas is so heated and compressed that it gives off X-rays.
Some accretion disks shine as brightly as a thousand galaxies. Those objects, called quasars, have been seen at the centers of very old galaxies, suggesting that quasars were plentiful in the early universe, but are now often dormant or much diminished in activity.
It has been difficult to know how common such objects are. The sky is lit in all directions with an X-ray glow or "background." But until recently, instruments have not been sensitive enough to find its specific sources.
Today, however, astronomers using Chandra, NASA's newly launched X-ray telescope of unprecedented resolution, announced the discovery that the background radiation comes from myriad individual sources, most of them galaxies presumably containing black holes at their core. One-third of those sources give off no visible light. "There are roughly 100 million [X-ray sources] over the entire sky," said Chandra scientist Richard Mushotzky of NASA's Goddard Space Flight Center.
Another group, using data from the Hubble Space Telescope, discovered black holes at the centers of three nearby galaxies, bringing to 20 the number of such objects identified. The apparent ubiquity of the holes implies that "the formation and evolution of galaxies are intimately connected to the presence of a central black hole" in each, said Douglas Richstone of the University of Michigan.
Which came first, the galaxy or the hole? "It's still an open question," Richstone said, "whether it was the chicken or the egg." But observations suggest that "quasars turned on faster than stars." It is possible, he said, that in the early universe huge agglomerations of matter formed, "but somehow managed not to fragment into stars. So you wind up with this lump of two million solar masses or so, which collapses to a black hole right away. Our results suggest that we see the black holes before we see the stars."
"It's likely," Richstone said, "that these black holes play a key role in the formation of galaxies." Astronomers think that our own Milky Way galaxy has a black hole of about 2.5 million solar masses at its core.
Andrew S. Wilson of the University of Maryland suspects that ultimately black holes will be found in "100 percent" of galaxies except for the "dwarf" variety. His group announced that, according to its new observations, about 30 percent of nearby galaxies contain "dying quasars or quasar relics." These are produced by black holes that are swallowing surrounding gas far more slowly than they once did.
Wilson's team used the National Science Foundation's Very Large Array and Very Long Baseline Array of radio telescopes to show that nearly all these galaxies have radio wave sources that are "very compact," less than a light year (about 6 trillion miles) wide.
"The only known way in which these radio sources can be produced is accretion into a supermassive black hole," Wilson said.
A black hole need not be surrounded by matter; it could form from the gravitational collapse of a fairly small star. But without the radiation generated by an accretion disk, it would be virtually impossible to see one.
Today, however, two international teams reported seeing "naked" black holes by detecting their effects on illumination from other sources. The gravitational field of even a small black hole is so strong that it bends light like a lens. So when a naked black hole passes between an observer and a distant star, the star will suddenly look brighter.