Black Hole Sound Waves



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



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Sept. 9, 2003: 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 traveled hundreds of thousands of light years
away from the cluster's central black hole.

Right: The <A HREF="http://antwrp.gsfc.nasa.gov/apod/ap980815.html">Perseus
cluster</A> of galaxies. Each fuzzy object is a galaxy. Unseen
is a vast cloud of hot gas filling the cluster. Near the center of it all
lies a supermassive black hole.


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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."





Above: Peering into the heart of the Perseus Cluster (left), the Chandra
X-ray Observatory detected sound waves rippling through the gas (right). [<A
HREF="http://chandra.harvard.edu/photo/2003/perseus/">more</A>]

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 center 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 center. 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.

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 <A
HREF="http://chandra.harvard.edu/chronicle/0402/cosmic_jets/index.html">jets of
material</A> 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.

Right: an illustration of cavities and sound waves in the hot gas filling the
Perseus cluster. [<A
HREF="http://chandra.harvard.edu/photo/2003/perseus/more.html">more</A>]

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.








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