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How an X-ray Telescope Detected Sound Produced by a Black Hole
September 16, 2003
Chandra X-ray Image of Perseus
The galaxy cluster in Perseus
is a colossal structure containing about 500 galaxies. Despite this impressive
size, there is a lot more mass between the cluster galaxies, in the form
of hot gas, than there is in the trillions of stars within these galaxies.
Only the elusive dark matter makes up a larger fraction of the total cluster
The hot gas in the Perseus cluster is very useful for tracing the power
of the supermassive black hole at the center of the cluster, and for studying
the growth of the large central galaxy. The observational challenge to astronomers
is that the 50 million degree gas is only detectable with X-ray telescopes.
Early X-ray telescopes, with less advanced optics, struggled to detect structure
in the cluster gas, especially near the central black hole. Now the much
sharper vision of the Chandra X-ray Observatory has revealed beautiful structures
in the cluster gas, culminating in the recent report, by British astronomers,
of evidence for sound waves generated by the supermassive black hole in
the cluster’s central galaxy.
Chandra Ripple Image of Perseus
Sound is a type of pressure wave that travels through a medium, such as
air, water, or the gas in a galaxy cluster. Bulk motion of the material
does not take place as the pressure waves move away from the source of the
sound. A good physical analogy is a ripple traveling along a slinky.
Sound is generated when an object makes a disturbance in a medium. Examples
include vibrations from our vocal chords, from a tuning fork or from a membrane
in a stereo speaker. In the Perseus galaxy cluster, scientists believe that
sound is generated in the following way: cavities or bubbles in the cluster
gas are blown out by jets from a supermassive black hole (located at the
center of the cluster). These cavities eventually push against the cluster
gas like a piston, generating a pair of sound waves which travel away from
the now slowly traveling cavities.
[View the animation
How is the Chandra X-ray Observatory able to detect these sound waves?
As mentioned above, the gas that pervades the Perseus cluster is very
hot, and glows in X-rays. If the gas is directly visible, then pressure
waves, if they are large enough, will also be visible, as "ripples" in
the cluster gas. This is what is happening in the Perseus cluster. Scientists
are able to see the sound waves, rather than hear them. By contrast, sound
waves are not seen in the earth's atmosphere because our eyes are only
sensitive to light at optical wavelengths, and air is much too cool to
glow at these wavelengths. If air is invisible, the pressure waves (sound)
passing through it will also be invisible.
Although the sound waves in the Perseus cluster are moving rapidly, the
distance to the cluster is so large (250 million light years) that the
motion of the waves is undetectable from Earth
What type of sound is being generated by the black hole?
If sound waves are produced at fixed, regular time intervals, then a
tone, or note is created, similar to the production of musical notes with
a tuning fork. The frequency (or pitch) of the note depends on the size
of the tuning fork: a small tuning fork moves more rapidly, generating
a closely spaced set of sound waves, corresponding to a high frequency
note. A larger tuning fork moves more slowly generating more widely spaced
pressure waves and a lower frequency note.
Because the cavities in the Perseus cluster are so large, the scale of
the sound waves generated is colossal. But, a single cavity from the black
hole only produces a single sound wave. The regular spacing of sound waves
visible in the top right of the cluster shows that cavities have been
generated by the black hole at regular intervals. Knowing the speed of
sound plus the measured distance between waves allowed the scientists
to estimate the interval between waves at just under 10 million years,
corresponding to a frequency of about one cycle per 300 million million
seconds, about 57 octaves below B-flat above middle C.
The sound waves provide a critical source of energy for keeping the cluster
gas from cooling too much
and prevent massive amounts of star formation in the central galaxy. Scientists
have calculated that the pitch and intensity of the sound generated by
the black hole would have had to have been roughly constant for about
2.5 billion years to offset cooling due to X-radiation.