Sounding out a new role for black holes.

It's a tone no one can hear, generated by a body no one can see, yet its reverberations may be huge. Astronomers reported this week that they have for the first time detected sound waves generated by a black hole.

The finding appears to solve a long-standing puzzle about the behavior of gas at the center of galaxy clusters. It also points to a previously unrecognized role for super-massive black holes: limiting how big a galaxy can become.

Andrew C. Fabian of the University of Cambridge in England and his colleagues made their discovery by examining the bright X-ray glow from gas that bathes the Perseus cluster of galaxies, 250 million light-years from Earth. Most of the gas in the cosmos lies between widely separated galaxies but emits so little radiation that it's difficult to detect. However, the immense gravity of a cluster squeezes and heats the gas so that it emits X rays.

Using a 53-hour-long observation of Perseus with NASA'S, orbiting Chandra X-ray Observatory, Fabians team found circular ripples in the cluster's gas, similar to those created when a rock is dropped into a pool of water. The team attributes the ripples to sound waves generated when jets emanating from Perseus' central black hole plow into the cluster's intergalactic gas.

"We [had previously] observed the prodigious amounts of light and heat created by black holes, now we have detected the sound," Fabian says. He announced his team's findings at a NASA briefing in Washington, D.C.

The ripples are separated by about 35,000 light-years--which produces a B-flat 57 octaves below middle C. The black hole's tone is far lower than the human ear can discern.

The note is the deepest ever detected in the universe, Fabian says. But the Perseus sound waves are more than a curiosity.

Astronomers haven't been able to explain why the center of Perseus and other galaxy clusters are as hot as 50 million kelvins, even though the gas there radiates energy away and should therefore be cold. Fabians team says the sound waves solve the mystery. When they're absorbed by the cluster gas, the waves unleash energy equivalent to 100 million supernova explosions. That's enough to keep the gas hot within the innermost 150,O00 light-years of the cluster, Fabian says.

To explain how the newly detected sound waves are generated, Fabian points to previous Chandra observations of the Perseus cluster, which had revealed two large bubbles extending away from the central black hole. Fabian's team proposes that the bubbles originated when jets of material from the black hole slammed into cluster gas. The inflation of the bubbles produced shocks that were converted to the pressure waves, or sound waves, which spread out in all directions, Fabian suggests. Sound waves that are stopped within the cluster not only heat its core but also may drive some material outward.

Fabian speculates that a similar outward push by sound waves occurs in some individual galaxies that house a supermassive black hole. Preliminary calculations indicate that sound waves would pass freely through the gas of a low-mass galaxy but would be absorbed by the gas of a heavier one, Fabian told Science News.

Because sound waves generated by a supermassive black hole may drive material out of these heavier galaxies, the black hole could limit the amount of material a galaxy can pack on, thereby restricting how big it can get. Because the sound waves can carry energy thousands of light-years from a supermassive black hole, they may in part account for its influence on the growth of galaxies (SN: 4/5/03, p. 214).

Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md., cautions that the proposed link between black hole sound waves and cluster heating requires special circumstances. To be absorbed, the sound waves must have just enough oomph to travel through much, but not all, of the cluster's gas. Also, Mushotzky notes that Fabian's scenario requires that black holes pump out jets over periods 10 to 50 times longer than theorists have suggested.

None of these potential problems necessarily negates the findings, Mushotzky adds. If Fabian's team is correct, he says, the new observations are "of fundamental importance in all of astronomy and cosmology."