A group of astrophysicists at Yale has calculated the fate
of a pair of supermassive black holes at the center of a galaxy, showing that
they spiral inward and coalesce quickly when a large amount of gas is present.

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.

EDITORS: Photos and a movie to illustrate this release can be obtained over the
Internet after 4:30 P.M. EST, May 26, 2003 at
http://phoenix.astro.yale.edu/coppi/bhmerge/