May 30, 2007: Astronomers using NASA's Chandra X-ray Observatory have found evidence for an "awesome upheaval" in a massive cluster of galaxies. A bright arc of ferociously hot gas extending more than two million light years requires one of the most energetic events ever detected.
"The huge feature we detected in the cluster combined with its high temperature (170 million oC) points to an exceptionally dramatic event in the nearby Universe," says Ralph Kraft of the Harvard-Smithsonian Center for Astrophysics, leader of a team of astronomers involved in this research. "While we're not sure what caused it, we have narrowed it down to a couple of exciting possibilities."
Above: In this side-by-side comparison, an apparently ordinary star field in optical light (left) is shown to be dramatically different when observed in X-rays (right). Chandra's image of 3C438, the central galaxy within a massive cluster, reveals evidence for one of the most energetic events in the local Universe.
The favored explanation is that two massive galaxy clusters are running into each other at about 4 million miles per hour. When hot clouds of gas in the two clusters meet, shock waves produce a sharp change in pressure along the boundary where the collision is taking place, giving rise to the observed arc, which resembles an titanic weather front.
"Although this would be an extreme collision, one of the most powerful ever seen, we think this may be what is going on," says team member Martin Hardcastle of the University of Hertfordshire in the United Kingdom.
One problem with the collision theory is that only a single peak in the X-ray emission is seen, whereas two would be expected. Longer observations with Chandra and the XMM-Newton X-ray observatories should help determine how serious this problem is for the collision hypothesis.
Another possible explanation is that the disturbance was caused by an outburst generated by matter falling into a supermassive black hole. In this scenario, the black hole would inhale most of the matter but expel some of it outward in a pair of high-speed jets, heating and pushing aside surrounding gas.
Such events are known to occur in this cluster. A galaxy named 3C438 near the center of the cluster is a powerful source of explosive activity--presumably due to a supermassive black hole. But the energy in these outbursts is not nearly large enough to explain the Chandra data.
Right: A radio map of 3C438 reveals jets spewing from the galaxy's core--a sign of explosive activity. [More]
"If this event was an outburst from a supermassive black hole, then it's by far the most powerful one ever seen," says team member Bill Forman, also from the Center for Astrophysics.
The phenomenal amount of energy involved implies a very large amount of mass swallowed by the black hole, about 30 billion times the Sun's mass consumed over a period of 200 million years. The authors consider this rate of black hole growth implausible.
"These values have never been seen before and, truthfully, are hard to believe," notes Kraft. Until these issues are sorted out, the awesome upheaval remains a mystery.
These results were presented at the American Astronomical Society meeting in Honolulu, HI, and will appear in an upcoming issue of The Astrophysical Journal. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center, Cambridge, Mass.
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