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Galaxies Collide And Binary Black Holes Result According To Rutgers Astronomers

June 6, 2000
Rutgers, The State University Of New Jersey
Rutgers astronomers have used large-scale computer simulations to generate compelling evidence for the the existence of binary supermassive black holes at the centers of merged galaxies. The findings were presented Monday, June 5 at the American Astronomical Society meeting in Rochester, N.Y.

NEW BRUNSWICK/PISCATAWAY, N.J. -- Astronomers from Rutgers are providing compelling evidence that suggests supermassive black holes may occur in pairs at the centers of many galaxies. A number of observations over the last three decades have hinted at the existence of such black hole binaries although the arguments have never been persuasive.

David R. Merritt, professor of physics at Rutgers, is presenting the results of large-scale computer simulations of the merger of two galaxies that support the binary black hole model. The theoretical studies, described today at the American Astronomical Society meeting in Rochester, N.Y., were a joint effort involving Merritt, postdoctoral associate Fidel Cruz and graduate student Milos Milosavljevic, all of Rutgers.

Most galaxies are observed to have supermassive black holes at their centers, a feature acknowledged to have been present since shortly after the Big Bang, the gigantic explosion that signaled the beginning of the universe. Furthermore, most large galaxies are known to have formed by the repeated mergers of smaller galaxies, a process still ongoing today but at a slower rate. The question, then, is what becomes of the black holes at the centers of a pair of galaxies as the galactic merger occurs?

"A majority of astrophysicists had previously assumed that the two black holes would rapidly coalesce following a galaxy merger," said Merritt. "Our work suggests that the black holes do fall rapidly to the center of the resulting merged galaxy, but then form a binary system that persists over time with some degree of stability. The black holes orbit about their common center of mass."

According to the results of the computer simulation studies, this binary system then gradually shrinks, as the two black holes interact gravitationally with passing stars. In effect, the black holes experience a frictional force from the stars, causing them to lose energy and come closer together. In the process, the black holes eject stars at high velocities from the center of the galaxy.

The Rutgers astronomers found that this ejection process is so efficient that the black hole pair soon finds itself isolated at the center of the galaxy, having scoured the galactic nucleus clean of stars. Once this happens, there are no longer any stars left to interact with the black holes, and their orbital decay comes to a stop. The two black holes then remain in orbit about each other with a separation of roughly one parsec, or three light-years, that remains nearly fixed for billions of years.

"Our work should motivate renewed efforts to either confirm or rule out the binary black hole model," said Merritt. He noted that the predicted one-parsec separation of the two black holes in a binary system -- while enormous in everyday terms -- is nevertheless small enough to be difficult to observe in all but the nearest galaxies. "In most galaxies, the black hole binary would appear as a single massive object. Only very high-resolution techniques, such as radio interferometry (using the combined signal from many smaller single-dish radio telescopes to electronically simulate the effect of a very large dish), have the potential to directly observe a black hole binary," he said.

While advocating the binary black hole model, Merritt does not rule out other dynamic hypotheses. "In some galaxies, the black hole binary might be induced to shrink more rapidly by the presence of large amounts of gas or some other disturbance," he said. "If the black holes approached each other very closely, it could result in an enormous burst of energy as the two supermassive black holes coalesced violently into one even more massive black hole."

But in many galaxies, the new study suggests, this dramatic coalescence might never take place. Merritt pointed out one interesting consequence of the persistence of black hole binaries: If a third galaxy should happen to merge with the first two before their black holes had coalesced, a black hole would be introduced into a nucleus that already contained two black holes. The resulting violent interaction between the three black holes would almost certainly end with the complete ejection of one or more of the black holes from the galaxy's center. In this way, "rogue" black holes might be produced that travel, undetected, between the galaxies.

EDITOR'S NOTE: For additional information, contact Dr. David Merritt while in Rochester at the Four Points Sheraton Hotel, (716) 546-6400; later at Rutgers,(732) 445-5742; at home, (732) 249-3942; by e-mail, or see

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