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'Monster' Black Holes Activate When Galaxies Collide

Enormous black holes, some of the most powerful sources of radiation in theuniverse, apparently switch on after galaxies collide, researchers have found.

The centers of as many as a tenth of all galaxies generate more energy thancan be explained by stars, with some of these "active galacticnuclei" releasing more radiation than the entire Milky Way galaxycombined, but from a space no larger than our solar system. Astronomers suspectthis energy is released when matter falls into giant, supermassive black holes that are up to billions oftimes the mass of our sun at these galaxies' cores.

"These monster black holes evolve in a way that is strongly related tothe amount of dark matter that surrounds them and that is intimately related tothe probability of galaxies to merge," said study lead author Nico Cappelluti, anastrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

The new study supports the idea that these active galactic nuclei arestimulated when two galaxies are drawn together by their mutual gravitationalattraction and ultimately merge to become one. The process likely shakes up matter and pushes material toward the center ofthe new mega-galaxy, creating a perfect feeding ground for a hungry black hole.

This concept has been proposed before, but the new research provides furtherobservational evidence that this is the case. [Photos:When galaxies collide.]

Black hole power activate!

To study these active galactic nuclei, researchers analyzed 199 galaxies.

They relied on hard X-ray data from NASA's Swift spacecraft, which providedunprecedented depth and details about X-ray sources, Cappellutisaid. Previous studies investigated active galaxies based on optical or softX-ray emissions, which miss a major part of the radiation generated bygalaxies' central black holes.

The black holes powering these active galactic nuclei were typically about300 million solar masses in size ? roughly 75 times that of the black hole atthe Milky Way's core. Their galaxies were usually about 200 billion solarmasses in size, residing in huge bubbles of dark matter 100 times as massive asthe entireMilky Way.

Galaxy collision culprit

By combining this observational data with theoretical predictions, thescientists found the most plausible scenario for the origin of these activegalactic nuclei involved galaxy collisions.

"We find it very exciting how the results of our data analysis matchthe prediction of computer simulations that assume that the black holes areswitched on by galaxy mergers," Cappelluti toldSPACE.com.

The active galactic nuclei appear to switch on roughly 700 million yearsafter two galaxies collide and merge "and shine brightly for the firstpart of their lives, where they gain most of their mass," he explained.

After roughly 200 to 500 million years, matter accretes onto the black holeswith lower and lower efficiency, essentially starving the black holes as thegas reservoirs around them become depleted.

"Today, they have grown to super-massive black holes with 100 to 1,000million solar masses and shine with moderately low luminosity compared to otheractive galaxies," Cappelluti said.

In the future, the researchers would like to see what relationship mightexist between the energy output of an active galactic nucleus and the mass ofthe dark matter bubble in which it lives.

The scientists detailed their findings in a paper to be published in an upcomingissue of The Astrophysical Journal.

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