WASHINGTON, D.C. -- NASA's Chandra X-ray Observatory has discovered "ghostly" relics of an ancient eruption that ripped through a cluster of galaxies. The find, announced today by scientists, suggests that galaxy clusters are the locales of powerful and recurring explosions, and may help explain why galaxy clusters act as giant cosmic magnets.
Chandra observed the galaxy cluster -- Abell 2597 -- on July 28, 2000. Using the spaceborne observatory's Advanced CCD Imaging Spectrometer (ACIS) instrument, the device was trained on the cluster for 40,000 seconds.
"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, a research scientist from Ohio University in Athens.
McNamara is part of a team of scientists that made the Chandra observation, announcing the discovery at a press conference held today at the American Astronomical Society (AAS) meeting now underway in Washington, D.C.
The ghost cavities "appear to be remnants of an old explosion where the radio emission has faded away over millions of years," McNamara said.
Extremely powerful explosions likely create ghost cavities, McNamara explained, 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.
A one-time event?
This explosion found by Chandra appears not to be 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," McNamara said. 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, he said.
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.
The Chandra X-ray observatory is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. The Smithsonian's Chandra X-ray Center in Cambridge, Massachusetts controls science and flight operations of the space-based telescope.
Launched and deployed in July 1999 by space shuttle Columbia astronauts, the Chandra X-ray Observatory is the most sophisticated X-ray observatory constructed to date. Prime contractor for the spacecraft is TRW, Inc. of Redondo Beach, California. Chandra's prime duty is to observe X-rays from high-energy regions of the universe, such as the remnants of exploded stars.
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