Black Hole Sound Waves

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

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.