It is thought that each galaxy has a supermassive black hole—one with somewhere between one million and one billion solar masses at its center—but only about one percent of these "light up" and emit a large amount of energy. New findings from NASA's Swift satellite have pinpointed the reason. It turns out that the bright black holes arise in galaxies 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 billion times the energy output of our sun, making them some of the brightest objects in the entire universe. Previously, it was theorized that the disruptive act of galactic collision could send a large amount of mass into these black holes, causing them to emit large amounts of energy. Simulations of galactic collisions run on supercomputers supported this, but observations proved difficult.

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

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

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

The Astrophysical Journal Letters, 2010. Paper set to appear in the July 20 edition.