Astronomers have long suspected that when galaxies collide, their central black holes could merge. A new computer model provides some of the most compelling evidence to date that such colossal collisions do indeed take place, about once a year.
The computer model is based on some actual observations: Some galaxies spew jets of energy in two directions along their axis of rotation. These jets are thought to be created by black holes, which researchers cannot see directly. In about 7 percent of the galaxies with jets, astronomers have seen evidence for sudden shifts in the jets' directions, based on observations with National Science Foundation's Very Large Array (VLA) radio telescope.
"Flipped jets suggest that the black hole has suddenly been realigned," says Ron Ekers of CSIRO's Australia Telescope National Facility in Sydney, Australia.
The new model, which shows how these jets might realign, was created by Ekers and David Merritt of Rutgers University and is discussed in the Aug. 1 online version of the journal Science. The researchers say their model is the strongest evidence to date that the black holes' mutual attraction ends in an embrace rather than an endless waltz.
Simulations show that when large galaxies merge, interactions between the black holes and stars will make the black holes sink towards the center of the combined galaxy. But as they approach the center the black holes will kick out nearby stars, switching off the mechanism that was drawing them together.
"Most astronomers assume that Nature finds a way to bring the black holes together, since we don't see strong evidence of binary black holes at the centers of galaxies," says David Merritt. "We now have solid evidence that the black hole mergers actually take place."
The study's most important implications are for supermassive black holes, which anchor many large galaxies and can contain the mass of millions or billions of stars.
The new model shows, however, that even a small black hole can pack quite a punch, knocking another hole up to five times more massive through a large angle.
Theorists still can't explain what draws the black holes together once they have thrown the stars out of their neighborhood. But when the gulf between the merging holes has shrunk to the size of the solar system, the holes start to radiate away energy as gravity waves, Merritt said. Then the holes slide inexorably towards fusion, spiraling together faster and faster.
Their final clinch releases an enormous burst of gravitational radiation.
Observations of galaxies with flipped jets, along with an estimated lifetime of 100 million years for the lobes of radio emissions they produce, suggests these giant collisions occur about once a year, the researchers say.
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