Black Hole "Sound" Heats a Galaxy Cluster

The supermassive black hole at the heart of the Perseus A galaxy cluster is singing a note with a voice as powerful as hundreds of millions of supernovae, astronomers declared on Wednesday at a NASA press conference. This announcement made the news media sit up and take notice ("Black Hole Hums Deepest Note Ever Detected," headlined CNN), but the description was a stretch; it would take a very broad-minded physicist to consider the effect anything like a musical tone. It may, however, solve a longstanding galaxy-cluster mystery.

Astronomers have known for years that galaxy clusters are filled with very thin gas that is so hot (tens of millions of degrees) that it glows in X-rays. Andrew C. Fabian (Institute of Astronomy, England) and seven colleagues pointed the Chandra X-Ray Observatory at the Perseus A galaxy cluster, 250 million light-years away, and took exposures totaling nearly 55 hours. In X-rays Perseus A is the brightest galaxy cluster in the sky, and the Chandra image of its hot gas is the sharpest ever taken. The picture shows much detail, including a subtle series of what may be pressure waves — sound waves — expanding outward.

At the center of the cluster lies the giant galaxy NGC 1275, which harbors a supermassive black hole that emits two giant jets of plasma. These jets have cleared out two bubbles in the cluster's hot gas. When the bubbles grow large enough they should detach and rise buoyantly through the gas, like bubbles in water (S&T: May 2002, page 20). This bubbling action is probably what sends the pressure ripples moving outward. The ripples are spaced tens of thousands of light-years apart, indicating a frequency of about one cycle per 10 million years.

If that can be called sound, it's the B-flat note 57 octaves below middle C, said Bruce Margon (Space Telescope Science Institute) at the press conference. Kim Weaver (NASA/Goddard Space Flight Center) blew a B-flat on a clarinet to demonstrate. In fact, as a look at the images shows, only a half dozen individual waves are visible, and they are unevenly spaced; they would be an incoherent rumble rather than a specific note.

However, just like ordinary sound, the waves should carry energy away from their source — and lots of it. Each ripple, the astronomers estimate, transports the energy equivalent of 100 million supernovae. "We're seeing a new way black holes can dissipate energy out into the universe," Margon said. And this may explain a big mystery concerning galaxy-cluster gas.

The temperature and density of the gas in a galaxy cluster indicates that it is self-supporting. That is, its own internal gas pressure holds it up in balance against the force of gravity. (Scientists call this "hydrostatic equilibrium.") But there's a problem with this picture. Toward the center of a galaxy cluster, the gas should be pulled into the most massive individual member galaxies. As the gas falls together inward this way, its density should increase, thereby allowing it to radiate away its heat more efficiently and grow cool — further promoting the inward flow.

But such "cooling flows" just aren't seen the way they should be. Moreover, the galaxies themselves don't seem to be collecting any such added gas from their surroundings. What's holding it back?

Heating by the sound waves could do the trick. The amount of energy is just about right. "Here may be the solution to this fundamental problem with the gas in galaxy clusters," commented Weaver. And there may be broader implications for galaxy formation as well. "The growth of massive galaxies may be truncated by this process," suggested Fabian.