Astronomers at the Institute of Astronomy in Cambridge, England, discovered the sound waves while analyzing the Chandra images of the Perseus cluster, an immense grouping of galaxies held in formation by the powerful tug of a supermassive black hole.
Andy Fabian, a professor at the Institute of Astronomy, said a close study of the fine detail collected by Chandra shows ripples in the X-ray pattern that are caused by sound waves excited by the energy from the black hole.
He said the sound produced by the black hole is a B flat, the same pitch as a key near middle C on the piano. But the song of the Perseus Black Hole is 57 octaves below that middle C. This is a tone frequency more than a million, billion times deeper than the limits of the human ear, said Fabian.
The voice of the black hole is detected by analyzing the pattern of X-rays coming from superheated gases in the Perseus galactic cluster, said Fabian.
Squeezed by the gravitational pull of the black hole and the galaxies in the cluster, gases are heated to 50 million degrees, hot enough to generate X-rays. A surge of sound waves adds heat energy, causing a slight change in the pattern of X-rays.
"Sound consists of pressure waves," said Fabian. "In the gas of the Perseus cluster, the higher pressure means more X-ray emission."
In effect, he said, the sound waves cause bright and dark emissions of X-rays moving in rings away from the black hole center like ripples on the surface of a pool.
"A three dimensional analogy is when a child takes a straw and blows into a glass of water, producing a sequence of bubbles," said Fabian. "That is like a sequence of sound waves."
The distance between sound waves determines the pitch. For instance, in a very high pitched sound, such as the extreme upper note of a violin, the sound waves are close together and cycle many times a second. Most sounds detectable by the human ear cycle very rapidly.
But the voice of the black hole could never be heard by the human ear because there are 10 million years between each of the sound waves, "clearly not within human experience," said Fabian.
Bruce Margon of the Space Telescope Science Institute, said the study by Fabian and his co-author Steve Allen, also of Cambridge, shows a new way that black hole use to dissipate energy.
"We've known that a black hole can give off energy as light and heat and now we are seeing a third way — sound," said Margon.
He said that the Perseus Black Hole may play only one note, but it has been playing for about three billion years.
"Although this symphony does not have a lot of variety, it is surely the longest lasting symphony that we know," said Margon.
Fabian said that discovery is prompting astronomers to look for telltale ripples in the superheated gas around other black holes.
It is possible, he said, that other galactic clusters are singing in other tones.
"We would expect that every cluster and group of galaxies has its own note," said Fabian. "So if you look at the whole universe, there are many tunes being played."