One of the more spectacular phenomena in the cosmos might just be the collision of supermassive black holes that accompanies the merger of galaxies. But the astronomical community has not had definitive proof that these black holes are actually coming together. For the first time, astronomers have now produced a convincing mathematical model that offers the strongest support to date for the idea that the black holes merge when their host galaxies do.

David Merritt of Rutgers, The State University of New Jersey, and Ron Ekers of the Australia Telescope National Facility in Sydney, Australia, have published a paper online in Science Express that supports this interpretation.

Their calculations demonstrated that when two black holes merge, the interaction will realign the larger one. They showed for the first time that a smaller hole could knock a bigger one, with five times the mass, out of kilter.

The big switch: jets (inset) from the core of radio galaxy NGC 326 appear to have changed direction suddenly, interpreted as a result of black holes merging. The jets initially pointed to the 10 o'clock and 4 o'clock directions. They now point to 2 o'clock and 8 o'clock. Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.; STScI (for the inset).

The realignment takes place with a sudden flip in the spin axis of the larger hole. It shows up, said Merritt and Ekers, as a sudden switch in direction of the jets of particles that shoot out along the black hole's spin axis. Images made with a radio telescope show both the old and the new paths, and the galaxy appears X-shaped.

Supermassive black holes have been found in the center of almost every galaxy where astronomers have looked. From a few million to a few billion times the size of our sun (or solar masses), they are thought to have formed from giant gas clouds or from the collapse of clusters of immense numbers of stars shortly after the Big Bang when the universe began.

Merritt, who leads the Supermassive Black Hole Research Group at Rutgers, is a theorist who has worked extensively on the interaction of black holes with galaxies. Ekers, a prominent radio astronomer, is the president-elect of the International Astronomical Union (IAU) and director of the Australia Telescope National Facility.

"Supermassive black holes may have collided in a surprisingly large number of galaxies, leaving their signatures plain to read," reported Merritt and Ekers. About 7 percent of known radio-emitting galaxies show their jets in this characteristic X-shaped pattern. Merritt and Ekers calculated that a large galaxy has the probability of being involved in a collision once every billion years. Based on this calculation, one of these spectacles is bound to take place somewhere in the universe each year.

"We have known about X-shaped galaxies for a long time, but until now we have never had a convincing explanation for them," said Merritt. "Most astronomers were fairly sure that black holes coalesce, but we now regard the X-shaped galaxies as the first 'smoking-gun' evidence. Our model demonstrates that these constitute solid evidence that the black hole mergers actually take place."