Violent Galaxy Collisions Power Mega Black Holes

Among the most luminous things in the universe are active galactic nuclei (AGN): gigantic black holes that can emit as much energy as 10 billion suns. Why these objects are so much brighter than ordinary supermassive black holes has long puzzled researchers. Now, astronomers have confirmed what theorists consider the likely explanation: that AGN derive their stupendous power from the merger of two galaxies. “We think we have the ‘smoking gun’ for merger-triggered AGN,” says team member and graduate student Michael Koss of the University of Maryland, College Park.

Most galaxies in the universe revolve around central black holes, which feed voraciously on galactic gas and dust and spew out radiation. But about 1% of those black holes shine orders of magnitude more intensely than the rest do. In the past few years, the orbiting Swift x-ray observatory has found dozens of these extraordinarily bright black holes—or AGN—in nearby galaxies by detecting the high-energy x-rays they emit.

Koss and colleagues used a ground-based optical telescope to follow up on 260 AGN identified by SWIFT. They observed that a surprisingly high number of them—as many as 25%—were inside pairs of galaxies in the process of merging. “We saw strong signs of disruption in these pairs, which indicated that a merger was ongoing,” says Koss, adding that individual galaxies in the pairs appeared severely distorted as the collision tore them to pieces. “Perhaps 60% of these galaxies will completely merge in the next billion years.”

The researchers conclude that the violent merging of two galaxies as their black holes spiral together into one supermassive black hole may be what powers AGN. The process could be driving vast amounts of gas and dust toward the coalescing black hole, in turn causing intense energy to billow out from the object, says Neil Gehrels, a physicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a member of the research team.

The findings, announced at a press conference this afternoon, are being published in the 20 June issue of The Astrophysical Journal.

“This is a very nice result” that allows researchers “to get a handle on how AGN are activated,” says Karl Gebhardt, an astronomer at the University of Texas, Austin, who was not involved in the study. “The large sample makes it fairly conclusive that AGNs and mergers are related.” However, he says, researchers still need to do more detailed studies to understand “whether the gas gets driven to the center from an actual merger event” or “simply from two galaxies passing by each other.”

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