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Interpreting the 'Song' Of a Distant Black Hole

Three color Chandra Observation of Perseus Cluster
Three color Chandra Observation of Perseus Cluster

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Black holes and X-rays

Astronomers in England have discovered a singing black hole in a distant cluster of galaxies. In the process of listening in, the team of astronomers not only heard the lowest sound waves from an object in the Universe ever detected by humans, but they've also discovered an important clue about the formation of galaxy clusters -- the largest structures in the cosmos.

Dr. Andrew Fabian and his colleagues at the Institute of Astronomy in Cambridge, England made their discovery using NASA's Chandra X-ray Observatory, an orbiting X-ray telescope that sees the Universe in X-ray light just as the Hubble Space Telescope sees it in visible light.

The black hole is situated in the center of a galaxy amid a group of thousands of galaxies collectively called the Perseus Cluster and located 250 million light years from Earth (meaning it took the light from these galaxies 250 million years to reach us). The sound waves coming from it are in the form of a single note, so rather than a song it is really a drone.

Sound waves in the Perseus Cluster
Sound waves in the Perseus Cluster
Using the piano keyboard's middle C note as a reference point for the middle of the piano key music range, Fabian's team determined the note is a B -flat. On a piano, the B-flat nearest middle C is located midway between 1/8th and 2/8th of an octave away. In musical terminology, this B flat is 1-1/2 steps from middle C.

The Perseus cluster black hole's B-flat, by contrast, is 57 octaves below middle C or one million, billion times lower than the lowest sound audible to the human ear! In terms of frequency (the time it takes a single sound wave to pass by), the lowest sounds a person can hear is 1/20th of a second. The Perseus black hole's sound waves have a frequency of 10 million years!

You may be wondering how a sound wave can travel through space. After all, sound waves require some sort of stuff to move through. This stuff, called a medium, can be air, water, or even solid rock. And space is thought of as lacking any medium because it is a vacuum.

In fact, space is not a pure vacuum but rather it contains stray bits of stuff -- gas atoms and dust of varying amounts. In the case of the Perseus cluster, the gas throughout it serves as the medium through which the sound waves coming from the central black hole travel.

Sound waves emanating from a black hole
Still image from animation of sound waves generated in perseus cluster
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The sound waves were indirectly detected using the Chandra telescope because the cluster gas is very hot and thus emits an especially energetic form of light called X rays, as well as less energetic visible light. And the gas is so hot because of the effects of the black hole.

More than an acoustic curiosity, these sound waves transport energy that keeps gas throughout the cluster warmer than it would otherwise be. These warmer temperatures, in turn, regulate the rate of new star formation, and hence the evolution of galaxies and galaxy clusters. This makes the findings far more significant for understanding the astrophysical evolution of the Universe.

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

Astronomers will now analyze other galaxy clusters for similar sound waves.

NASA's Goddard Space Flight Center