Sound waves 57 octaves lower than middle-C are rumbling away from a supermassive black hole in the Perseus cluster.

by Ron Koczor

Astronomers using NASA's Chandra X-ray Observatory have found, for the first time, sound waves from a supermassive black hole. The "note" is the deepest ever detected from any object in our Universe. The tremendous amounts of energy carried by these sound waves may solve a longstanding problem in astrophysics.

The black hole resides in the Perseus cluster of galaxies located 250 million light years from Earth. In 2002, astronomers obtained a deep Chandra observation that shows ripples in the gas filling the cluster. These ripples are evidence for sound waves that have travelled hundreds of thousands of light years away from the cluster's central black hole.

Earlier observations had revealed the prodigious amounts of light and heat created by black holes. "Now we have detected their sound, too," says Andrew Fabian of the Institute of Astronomy in Cambridge, England, and the leader of the study.

In musical terms, the pitch of the sound generated by the black hole translates into the note of B flat. But, a human would have no chance of hearing this cosmic performance because the note is 57 octaves lower than middle-C. For comparison, a typical piano contains only about seven octaves. At a frequency over a million billion times deeper than the limits of human hearing, this is the deepest note ever detected from an object in the Universe.

"The Perseus sound waves are much more than just an interesting form of black hole acoustics," says Steve Allen, also of the Institute of Astronomy and a co-investigator in the research. "These sound waves may be the key in figuring out how galaxy clusters, the largest structures in the Universe, grow."

more Peering into the heart of the Perseus Cluster (left), the Chandra X-ray Observatory detected sound waves rippling through the gas (right)

For years astronomers have tried to understand why there is so much hot gas in galaxy clusters and so little cool gas. Hot gas glowing with X-rays ought to cool because X-rays carry away some of the gas' energy. Dense gas near the cluster's centre where X-ray emission is brightest should cool the fastest. As the gas cools, say researchers, the pressure should drop, causing gas from further out to sink toward the centre Trillions of stars ought to be forming in these gaseous flows.

Yet scant evidence has been found for flows of cool gas or for star formation. This forced astronomers to invent several different ways to explain how gas contained in clusters remained hot. None of them were satisfactory.

Black hole sound waves, however, might do the trick.

more An illustration of cavities and sound waves in the hot gas filling the Perseus cluster.

Previous Chandra observations of the Perseus cluster reveal two vast, bubble-shaped cavities extending away from the central black hole. These cavities have been formed by jets of material pushing back the cluster gas. The jets, which are a counter-intuitive side effect of the black hole gobbling matter in its vicinity, have long been suspected of heating the surrounding gas. But the exact mechanism was unknown. The sound waves, seen spreading out from the cavities in the recent Chandra observation, could provide this heating mechanism.

A tremendous amount of energy is needed to generate the cavities, as much as the combined energy from 100 million supernovas. Much of this energy is carried by the sound waves and should dissipate in the cluster gas, keeping the gas warm and possibly preventing a cooling flow. If so, the B-flat pitch of the sound wave, 57 octaves below middle-C, would have remained roughly constant for about 2.5 billion years.

Perseus is the brightest cluster of galaxies in X-rays, and therefore was a perfect Chandra target for finding sound waves rippling through the hot cluster gas. Other clusters show X-ray cavities, and future Chandra observations may yet detect sound waves in those clusters, too.