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Embargoed for Release: 10:30 a.m. EST, May 26, 2003

Astrophysicists Predict Rapid Merging of Black Holes in Colliding
Galaxies

New Haven, Conn. -- 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. 

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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/

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