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Galactic pile-ups feed supermassive energy output

Violent collisions 'light up' black holes

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NASA's Swift satellite appears to have confirmed one reason why a small percentage of supermassive black holes throw out vastly elevated levels of energy: it's provoked by violent collisions between galaxies.

NASA explains that one per cent of such black holes - weighing in at "between a million and a billion times the Sun's mass" - exhibit such behaviour, emitting "10 billion times the Sun's energy".

These phenomena, dubbed "active galactic nuclei" (AGN), were theoretically shown to be caused by galactic pile-ups, and Swift's Burst Alert Telescope (BAT) has since 2004 been keeping an eye out for them in hard X-ray wavelengths (0.10 to 0.010 nm).

Hard X-rays are capable of penetrating solid material, which has proved essential for pinpointing AGN. NASA elaborates: "Until Swift's hard X-ray survey, astronomers never could be sure they had counted the majority of the AGN. Thick clouds of dust and gas surround the black hole in an active galaxy, which can block ultraviolet, optical and low-energy, or soft X-ray, light. Infrared radiation from warm dust near the black hole can pass through the material, but it can be confused with emissions from the galaxy's star-forming regions."

The upshot is "dozens of previously unrecognized systems" displaying AGN behaviour, with roughly a quarter "in mergers or close pairs". Michael Koss of the University of Maryland, lead author of the findings which will appear in the 20 June issue of The Astrophysical Journal Letters, said: "The Swift BAT survey is giving us a very different picture of AGN. Perhaps 60 percent of these galaxies will completely merge in the next billion years. We think we have the 'smoking gun' for merger-triggered AGN that theorists have predicted."

By way of illustration, NASA has handily offered the "optical counterparts of many active galactic nuclei (circled)", captured by the 2.1-meter telescope at Kitt Peak National Observatory in Arizona (big version here):

Optical view of merging galaxies with AGN circled. Pic: NASA

NASA has more on the Swift BAT survey here. ®

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