August 1, 2002
Scientists Detect "Smoking Gun" of Colliding Black Holes
Images from the National Science Foundation's Very Large Array (VLA) radio telescope have uncovered compelling evidence that supermassive black holes at the hearts of large galaxies collide when their host galaxies merge.
"What we have found is the smoking gun for black hole collision in merging galaxies," said David Merritt, an astrophysicist at Rutgers University in New Brunswick, New Jersey. Merritt and his colleague Ronald D. Ekers of CSIRO's Australia Telescope National Facility in Sydney, Australia, present their findings in the August 1 edition of Science Express.
When large galaxies merge, current models predict that their central black holes would sink toward the center of the combined galaxy and form a binary pair. "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 Merritt. "What we have found in the VLA data is the first direct evidence that the black holes actually do coalesce."
The evidence for these mergers comes from the "jets" of radio emitting particles that shoot from the cores of large galaxies. These jets are oriented parallel to the spin axis of the supermassive spinning black hole and are generated by a disk of material being pulled into it.
Images taken with the VLA reveal that about 7 percent of radio emitting galaxies appear to have their jets flipped, forming what is known as an "X-type" radio source, so named because of the "X" shape of the radio lobes. "Flipped jets suggest that the black hole has suddenly been realigned," said Ekers.
These features are formed, the astronomers believe, when black holes collide in a cosmic version of a demolition derby. "Black holes are so large and so massive," said Merritt, "that the only thing we can imagine that would have enough force to realign them is another black hole." The astronomers' calculations show for the first time that even a small black hole can significantly impact the orientation of another one up to five times more massive.
The mechanism of how these objects collide, however, has not been well understood. Theory predicts that as the black holes draw near, they kick out the surrounding stars in their neighborhood, which initially provided the braking power to bring them together. Merritt believes that when the distance between the black holes shrinks to about the size of the solar system, they start to radiate away energy as gravity waves. This then brings the black holes closer and closer, causing them to spin faster and faster, until they eventually collide in an enormous burst of gravitational radiation.
Considering the large population of radio galaxies that appear to have flipped jets, and the estimated 100 million-year lifetime of the jets, the astronomers believe that these collisions happen somewhere in the Universe at the rate of about one a year. Proposed space-based gravitational wave detectors, therefore, should be able to detect these dramatic events.
The astronomers used existing images from the VLA to produce their results. "The VLA has dominated the whole field of radio galaxy studies. In this current research, the quality of the images has been of crucial importance. In each case, the evidence for the black hole having suddenly been realigned has been the detailed morphology of the radio galaxy. We've used the results of many observing programs using the VLA, involving many different groups from around the world," Ekers said.