Computer simulations predict that when large galaxies collide, their black holes will be drawn toward the center of the new, combined galaxy. But what happens next is unclear. Although many models predict the two entities should coexist as a pair, evidence of binary black holes at the center of galaxies is lacking. Now the results of a new mathematical model, published online today by the journal Science, suggest that the black holes in fact merge--and leave behind a telltale radio signature.

David Merritt of Rutgers University and Ron D. Ekers of the Australia Telescope National Facility studied images collected by the Very Large Array (VLA) radio telescope. Most radio-active galaxies produce two jets of emissions spouting into space in opposite directions, but approximately 7 percent are so-called X-shaped galaxies. In these, the two jets adopt an irregular, or winged, configuration. According to Merritt, astronomers "have known about X-shaped galaxies for a long time, but until now we have never had a convincing explanation for them." Because the jets are thought to mark the spin axis of the galaxy's central black hole, Merritt and Ekers posited that the flipped jets of X-shaped galaxies resulted from an adjustment to their black holes. "Black holes are so large and so massive," Merritt explains, "that the only thing we can imagine that would have enough force to realign them is another black hole."

The team's calculations bore this prediction out. They determined that regardless of how quickly a black hole is spinning, a merger with a counterpart even one fifth its size would be sufficient to affect its orientation. Although the mechanism for these mergers remains uncertain, the scientists calculate that at least one occurs--somewhere in the universe--each year. "Most astronomers were fairly sure that black holes coalesce, but we now regard the X-shaped galaxies as the first 'smoking-gun' evidence," Merritt says. "Our model demonstrates that these constitute solid evidence that the black hole mergers actually take place."