X rays unveil secret lives of black holes

Supermassive black holes lead busy lives behind veils of dust that keep much of their activity under wraps. New X-ray observations challenge the notion that these cannibals, which reside at the cores of galaxies, finished growing soon after their host galaxies formed. Instead, these gravitational beasts–1 million to 1 billion times the mass of the sun–may pack on weight much more gradually, gobbling surrounding gas and stars for up to 2 billion years.

Artist’s conception of a supermassive black hole. NASA/Univ. of Hawaii Inst. of Astronomy

Visible-light counterparts of 13 X-ray emitters believed to be supermassive black holes. These galactic black holes are grouped according to their distance from Earth, with the closest depicted at lower left. Barger, Cowie et al.

These and other studies, all reported last month, suggest new ways to find black holes at the cores of galaxies and measure their mass. Supermassive black holes are the big brothers of stellar black holes, which typically weigh about 10 times the sun’s mass.

Most of the new conclusions trace to data gathered by two satellites, NASA’s Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mirror Mission (XMM)�Newton observatory. These satellites are the first to produce high-resolution images and spectra of the cosmos by detecting high-energy X rays (SN: 10/21/00, p. 266). Because energetic X rays easily penetrate dust, astronomers can now identify and study a host of heavenly objects–including supermassive black holes–that are too faint to be seen in visible light. “We’ve entered a new era in the study of black holes,” says Richard F. Mushotzky of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Not all black holes hide behind dust. Some fuel quasars, the beacons of visible light that rank among the most brilliant objects in the cosmos. Black holes power quasars by converting gravitational energy–the energy gained as they suck in gas and dust–into radiation. The more a black hole eats, the more radiation it generates.

Although almost every large galaxy in the nearby reaches of the universe seems to have a black hole, less than 1 percent have quasars. This led researchers to suggest that quasars–and the black holes that power them–are active for only a blip of cosmic time, about 10 million years.

X-ray studies, however, suggest that’s far from the whole story. By using quasars to estimate how long black holes stay active, astronomers overlooked the vast majority of galaxies with active black holes because they aren’t easily detected in visible light (SN: 1/15/00, p. 36), Mushotzky notes.

Using Chandra to examine several hundred optically bright galaxies, a team led by Amy J. Barger of the University of Hawaii in Honolulu and the University of Wisconsin in Madison found that about 10 percent of the galaxies in the sample emit a high concentration of energetic X rays from their core. The strength of the X rays suggests that these galaxies contain supermassive black holes that are eating voraciously.

Extrapolating from these results, Barger suggests that all luminous galaxies go through a phase in which their cores show intense X-ray emission–a signpost that they harbor a black hole–and that at any time, about 10 percent of the black holes are active diners. In follow-up studies, Barger and her colleagues used visible-light and near-infrared telescopes to measure the distances to 13 of the 20 X- ray�emitting galaxies. Many reside less than 6 billion light-years away, but one is so distant that the light now reaching Earth left the galaxy when the universe was only one-fifth its current age.

The team found that in each interval of cosmic time examined–from the present back through some 10 billion years–roughly the same percentage of black holes, about 10 percent, were active feeders. If the universe is now 14 billion years old, supermassive black holes probably remain active, putting on significant weight, for about 1.4 billion years, calculate Barger and her colleagues, including Mushotzky and Lennox L. Cowie of the University of Hawaii.

If black holes take so long to grow, they might not wield as strong an influence on galaxy formation as astronomers had thought, Barger says. Her team’s observations indicate that at least 15 percent of all supermassive black holes accumulated most of their mass over the past 6 billion years, well after their host galaxies had fully developed. She reported the findings last month at the Texas Relativistic Astrophysics Symposium in Austin.

Mushotzky cautions that the number of galaxies in the team’s study is relatively small. Science News has learned that a team at Pennsylvania State University in University Park also finds that some 10 percent of optically bright galaxies show X-ray emission indicative of an active black hole.

“Even though we have more Chandra data and X-ray sources than [Barger’s team] . . . we are being more cautious about deducing black hole growth times, et cetera,” notes Neil Brandt, a member of the Penn State team.

In other reports at the Texas meeting, researchers described new methods for searching for supermassive black holes and determining their mass. One group presented evidence that an X-ray fingerprint can reveal the presence of black holes and indicate their mass.

X-ray spectra taken with the XMM-Newton observatory reveal emissions from iron atoms circling black holes. The proximity of the atoms to the black hole alters the width and location of peaks in their spectra. G�nther Hasinger of the Astrophysical Institute in Potsdam, Germany, and his colleagues used these telltale distortions to find evidence of black holes in two distant galaxies.

“The way the iron line is distorted . . . tells us that the mass concentration [responsible for the altered spectra] is so high that it can only exist in the form of a black hole,” he says. Hasinger estimates that by searching for the characteristic iron-emission lines, XMM-Newton can detect several hundred previously unknown black holes.

The iron spectra also contain information that enables astronomers to measure the distance to a black hole in a galaxy too faint or dusty for its visible-light spectra to be detected.

Astronomers can use fluctuations in the intensity of the iron emissions to estimate the mass of the black hole. XMM-Newton only detects such variability in the iron spectra in far brighter, nearby galaxies. Future X-ray observatories are being designed to record fluctuations from more distant galaxies, which hail from a time when the universe was much younger, Hasinger says.

Using this technique to weigh supermassive black holes that existed at different times, astronomers could directly determine how rapidly these beasts grow and if they were significantly smaller in the past, says Andrew C. Fabian of the University of Cambridge in England.

Other techniques described last month, while more speculative, may eventually enable astronomers to measure the masses of black holes going back to the era when galaxies first formed. These methods require the presence of quasars.

In one technique, researchers monitor the gas clouds that orbit a supermassive black hole and its quasar. If the quasar varies its brightness, so do the surrounding clouds. An observer detects the variation in the quasar first, and the time delay indicates the distance between the clouds and the black hole. Combining this information with the speed of the clouds, astronomers can determine the black hole’s mass, note Karl Gephardt and John Kormendy of the University of Texas at Austin. Other researchers have recently confirmed this method by weighing black holes of known mass in nearby galaxies.

A second method relies on the observation that the brightness of a gas cloud reflects its distance from the quasar. From this relationship and the velocity of the clouds, several teams of astronomers have begun calculating the mass of nearby black holes. If the techniques can be applied to the most distant quasars, astronomers will be able to probe the growth of black holes in the early universe.

“We have entered a golden age in observational studies of black hole accretion and growth,” says Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “The next decade is likely to be exciting.”

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