Look out if black holes collide

When black holes collide, look out! An enormous burst of gravitational radiation results as they violently merge into one massive black hole. The "kick" that occurs during the collision could knock the black hole clear out of its galaxy. The consequences of such an intergalactic collision are explained by David Merritt, professor of physics, Rochester (N.Y.) Institute of Technology, in the study "Consequences of Gravitational Radiation Recoil."

Virtually all galaxies are believed to contain supermassive black holes at their centers. According to current theory, galaxies grow through mergers with other galaxies. When this occurs, the central black holes form a binary system and revolve around each other, eventually coalescing into one. The coalescence is driven by the emission of gravitational radiation, as predicted by physicist Albert Einstein's Theory of Relativity.

Merritt and his colleagues determined how fast a black hole has to move to escape a galaxy's gravitational field completely. They found that larger and brighter galaxies have stronger gravitational fields and would require a bigger kick to eject a black hole than the smaller systems. Likewise, less forceful impacts could jar the black hole out of its home at the center of a galaxy, only to later rebound back into position.

The kicks call into question theories that would grow supermassive black holes from hierarchical mergers of smaller black holes, starting in the early universe. "The reason is that galaxies were smaller long ago, and the kicks would easily have removed the black holes from them" Merritt theorizes that it is more likely that supermassive black holes attained most of their mass through the accretion of gas, and that mergers with other black holes took place only after the galaxies had reached roughly their current sizes.

"We know that supermassive black holes exist at the centers of giant galaxies like our own Milky Way," he explains. "But as far as we know, the smaller stellar systems do not have any black holes. Perhaps they used to, but they were kicked out."

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