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Holey union. An x-ray image of two black holes superimposed on an optical light image of NGC 6240.

A Galaxy With Two Hearts

Astronomers have sighted evidence of two large black holes spiraling toward an eventual collision in the center of a nearby galaxy. The discovery--made by an international team of astronomers using data from the orbiting Chandra X-ray Observatory--has confirmed astronomers' long-held suspicions that the black holes at the hearts of many galaxies might come in pairs.

The galaxy that harbors the double black hole, NGC 6240, lies 400 million light-years from Earth. In 1983, astronomers observing NGC 6240 in visual light found that its shape is strongly distorted--an indication that it consists of two galaxies that have collided. What really piqued their curiosity, however, was that the galaxy radiated enormous amounts of power at longer wavelengths, in the infrared part of the spectrum.

Only two mechanisms can explain such huge infrared emissions. NGC 6240 might be alive with starbursts, swarms of newly forming stars. Alternatively, it might harbor an active galactic nucleus (AGN)--an enormous engine that blasts out x-rays as matter falls toward a black hole in the galaxy's center. Dust clouds near the core of the galaxy would absorb x-rays and reradiate the energy in the infrared.

The new observations reveal several telltale signs that the x-ray sources are black holes, says team member Stefani Komossa, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. First, they are very intense and concentrated, and they are emitting extremely high-energy x rays--hallmarks of AGNs but not of starbursts. When the team looked closely at the distribution of x-rays, they discovered two sources of x-rays separated by 3000 light-years. At this distance, it takes millions of years for the two holes to rotate around their common center. Over hundreds of millions of years, the two bodies will spiral toward each other, giving off energy in the form of gravitational waves, and ultimately merge.

Such mergers might explain why some galaxies don't show an increased concentration of stars toward the center, says Astronomer Royal Martin Rees of the University of Cambridge, U.K. "In the process of merging, the binary [black holes] would have kicked out the stars from the center."

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