Galaxy collisions give black holes a healthy glow

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It is thought that eachgalaxy has a supermassive black hole—one with somewhere between onemillion and one billion solar masses at its center—but only aboutone percent of these "light up" and emit a large amount of energy. Newfindings from NASA's Swift satellite have pinpointed the reason. It turns out that the bright black holes arise ingalaxies that are in the process of colliding.

The brightest of these supermassive black holes, known as active galactic nuclei (AGN), can emit as much as 10 billiontimes the energy output of our sun, making them some of the brightestobjects in the entire universe. Previously, it was theorized that thedisruptive act of galactic collision could send a large amount of massinto these black holes, causing them to emit largeamounts of energy. Simulations of galactic collisions run onsupercomputers supported this, but observations proved difficult.

The thick clouds of dust andgas that exist around galactic nuclei block a large portion of theelectromagnetic spectrum, including UV, optical, and low energy X-rays.Infrared radiation gets through, but it's difficult to separate the black hole's signal fromnearby star-forming regions of the galaxy. It took Swift's Burst AlertTelescope, which can map hard (high-energy) X-rays, to probe these concealedregions of AGNs.

The hard X-ray survey carriedout by Swift took place over the course of a few years, and now represents the most detailed survey of the sky at these energylevels. It has allowed astronomers to gather a complete census of allAGNs that exist within 650 million light years of Earth, and uncovereddozens of previously unknown AGNs.

The high portion of galaxies inthe process of merging, or in gravity-linked pairs, pointed to the cause ofbright supermassive black holes. Perhaps 60 percent of the galaxies surveyed "will completely merge in the next billionyears," said Michael Koss, the study's lead author. "We think we have the 'smoking gun' for merger-triggered AGN thattheorists have predicted."

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