The work presented today at the American Astronomical Society meeting in Nashville, Tenn. consists of a series of numerical simulations of an orbiting pair of black holes embedded in a massive gas cloud. Such gas clouds are often observed at the centers of ultraluminous infrared galaxies, objects that are interpreted as mergers in progress.

Doctoral student Andres Escala of Yale and the Universidad de Chile performed the study under the supervision of Paolo Coppi and Richard Larson, professors of astronomy at Yale.

Supermassive black holes are a common phenomenon in the universe since nearly every large galaxy has one at its center. Large galaxies are believed to form through a series of mergers of smaller galaxies, many of which may have contained their own central black holes. It is important to understand, said Escala, whether these central supermassive black holes merge when the galaxies merge. In the merger scenario, this is presumed to happen because most large galaxies contain a single central supermassive black hole.

"Our work explores this question and suggests that, in a merger of galaxies containing a reasonable amount of gas, the answer is yes and the central supermassive black holes coalesce shortly after the galaxies merge," Escala said.

"The orbiting black holes are predicted to spiral together and sink toward the center because of the gravitational drag effect produced by the gas, which tries to follow the motion of the black holes but always lags behind," said Larson.

The simulations show that the black holes spiral inward and form a massive close binary system at the center of the galaxy. Once the binary has formed, it creates an ellipsoidal enhancement in the density of the surrounding gaseous medium that trails behind the binary. "The decelerating torque exerted by this trailing ellipsoidal enhancement makes the black holes continue to approach each other," Coppi said.

This result differs considerably from that obtained when the background is made entirely of stars instead of gas because the binary then acts as a baseball bat that knocks out all the stars that pass too close to it. "The ejection of the stars produces a hole in the surroundings of the binary, causing the coalescence to stall when the binary is formed," Coppi said. In the new simulations with gas, however, the gas is not ejected but remains concentrated near the black holes.

Because of this gas and its drag, the rapidly orbiting black holes come close enough that gravitational radiation becomes important and eventually causes their final coalescence. "This final coalescence of the black holes will produce a burst of gravitational waves that will be observable out to a great distance," said Escala. "Such bursts will be detectable with LISA, the National Aeronautics and Space Administration's space laser interferometer that is expected to be launched in 2010."

The detection of such gravitational waves would be a major test of Einstein's theory of general relativity, and it would also provide direct evidence for the predicted merging of supermassive black holes in galactic nuclei.

This work was supported by the Andes Foundation under the Yale-Universidad de Chile Collaborative Program and by the Chilean FONDAP project 15010003.