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Mergers between black holes are thought to be one way they grow. A handful of similar setups have been observed in which black holes appear inevitably on a merger course. This pair, at the center of a galaxy cluster called Abell 400, was known to be close but their fate hadn't been determined.
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Each of the black holes in Abell 400 is ejecting a pair of oppositely directed jets of superheated gas called plasma. The movement of the black holes through gas in the galaxy cluster causes the plasma jets to be swept backward.
"The jets are similar to the contrails produced by planes as they fly through the air on Earth," Sarazin said. "From the contrails, we can determine where the planes have been, and in which direction they are going. What we see is that the jets are bent together and intertwined, which indicates that the pair of supermassive black holes are bound and moving together."
When the objects merge several million years from now, Einstein's theory of relativity predicts they will emit a burst of gravitational waves. Similar mergers could soon be detected by NASA's planned Laser Interferometer Space Antenna (LISA).
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more:
http://www.cnn.com/2006/TECH/space/04/06/black.holes/index.html