Yale Science News Press Release

CONTACT: Jacqueline Weaver 203-432-8555

Embargoed for Release: 10:30 a.m. EST, May 26, 2003; Figures with extended captions and movie files follow release text.

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 ultra luminous 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.


PAGE TWO                                                                           (Black Holes Coalesce)

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: This material was presented to the American Astronomical Society meeting in Nashville, Tennessee. Fig. 1 Shows a simulation of two black holes at the center of a dense gas cloud, which are orbiting around each other and beginning to spiral together Fig. 2 After a close binary pair of black holes has formed, the gas develops a trailing ellipsoidal density enhancement, and this produces a decelerating torque that continues to drive the black holes together. Fig. 3 This composite view shows (upper left) a Hubble Space Telescope optical picture of the ‘cosmic train wreck’ NGC 6240, two galaxies that have collided and are in the process of merging, and (lower right) a Chandra X-ray image of the central parts of this system. Movie: This movie shows a sequence consisting of (1) a computer simulation of the merging of two galaxies (2) HST and Chandra views of the merging system or ‘cosmic train wreck’ and the two black holes near its center and (3) a simulation of two black holes spiraling together at the center of a dense gas cloud. PHOTO CREDIT: Fig. 3 and the movie contain a montage of photos by X-Ray: NASA/CXC/MPE/S. Komossa et al; Optical: NASA/STScI/R.P. van der Marel & J. Gerssen. Galaxy collision simulation in movie: Josh Barnes (University of Hawaii)/ John Hibbard (NRAO). The movie and figures can be obtained over the Internet after 4:30 P.M. EST, May 26, 2003 via:

http://www.astro.yale.edu/coppi/bhmerge (immediately below)


=============================================FIGURES AND MOVIE========================================

GIF Version of Figure 1   TIFF Version of Figure 1


GIF Version of Figure 2   TIFF Version of Figure 2


PNG Version of Figure 3  JPEG Version of Figure 3   GIF Version of Figure 3  TIFF Version of Figure 3


Here is an MPEG version of the simulation Figures 1 and 2 were drawn from. This movie shows a sequence consisting of (1) a computer simulation of the merging of two galaxies (2) HST and Chandra views of an actual merging galaxy system (a ‘cosmic train wreck’) with two supermassive black holes inferred to be very close together and near the center of the system (3) a simulation of two black holes spiraling together at the center of a dense gas cloud, and (4) a trailer depicting the gravitational radiation emitted by the two black holes as they finally coalesce into one black hole.

low resolution version (5.4 MB)      high resolution version (10 MB)