“Ghostly” relics of an ancient eruption that tore through a cluster of
galaxies were recently uncovered by NASA’s Chandra X-ray Observatory.
The discovery implies that galaxy clusters are the sites of enormously
energetic and recurring explosions, and may provide an explanation why
galaxy clusters behave like giant cosmic magnets.

“Chandra’s image revealed vast regions in the galaxy cluster Abell 2597
that contain almost no X-ray or radio emission. We call them ghost
cavities,” said Brian McNamara of Ohio University in Athens today
during a press conference at the American Astronomical Society meeting
in Washington. “They appear to be remnants of an old explosion where
the radio emission has faded away over millions of years.”

The ghost cavities were likely created by extremely powerful
explosions, due to material falling toward a black hole millions of
times more massive than the Sun. As the matter swirled around the black
hole, located in a galaxy near the center of the cluster, it generated
enormous electromagnetic fields that expelled material from the
vicinity of the black hole at high speeds.

This explosive activity in Abell 2597 created jets of highly energetic
particles that cleared out voids in the hot gas. Because they are
lighter than the surrounding material, the cavities will eventually
push their way to the edge of the cluster, just as air bubbles in water
make their way to the surface.

Researchers also found evidence that this explosion was not a one-time
event. “We detected a small, bright radio source near the center of the
cluster that indicates a new explosion has occurred recently,” said
team member Michael Wise of the Massachusetts Institute of Technology
in Cambridge, “so the cycle of eruption is apparently continuing.”

Though dim, the ghost cavities are not completely empty. They contain a
mixture of very hot gas, high-energy particles and magnetic fields —
otherwise the cavities would have collapsed under the pressure of the
surrounding hot gas.

“Ghost cavities may be the vessels that transport magnetic fields
generated in a disk surrounding a giant black hole to the cluster gas
that is spread over a region a billion times larger,” said McNamara. If
dozens of these cavities were created over the life of the cluster,
they could explain the surprisingly strong magnetic field of the
multimillion-degree gas that pervades the cluster.

Galaxy clusters are the largest known gravitationally bound structures
in the universe. Hundreds of galaxies swarm in giant reservoirs of
multimillion-degree gas that radiates most of its energy in X-rays.
Over the course of billions of years some of the gas should cool and
sink toward a galaxy in the center of the cluster where it could
trigger an outburst in the vicinity of the central massive black hole.

Chandra observed Abell 2597 on July 28, 2000,for 40,000 seconds with
the Advanced CCD Imaging Spectrometer (ACIS) instrument. Pennsylvania
State University, University Park, and MIT developed the instrument for
NASA. In addition to a group of astronomers from the Space Telescope
Science Institute, Baltimore, and the University of Virginia,
Charlottesville, the team included: Paul Nulsen, University of
Wollagong, Australia; Larry David, Harvard-Smithsonian Center for
Astrophysics, Cambridge, Mass.; Chris Carilli, National Radio Astronomy
Observatory, Socorro, N.M.; and Craig Sarazin, University of Virginia.

NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program, and TRW, Inc., Redondo Beach, Calif., is the prime
contractor for the spacecraft. The Smithsonian’s Chandra X-ray Center
controls science and flight operations from Cambridge, Mass.

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