Galactic hit-and-run leaves trail of destruction

日期:2017-09-24 01:00:07 作者:党噻滕 阅读:

By Rachel Courtland (Image: Tomer Tal/Jeffrey Kenney/Yale/NOAO/AURA/NSF) A trail of gas connecting two galaxies shows they once suffered a high-speed collision. The event could help explain why many galaxies cannot form new stars. One victim of this intergalactic accident is the “disturbed” spiral galaxy NGC 4438, which sits roughly 50 million light years from us in the Virgo galaxy cluster. The spiral arms of this galaxy look mangled, and it is unusually poor in gas compared with similar spirals. Astronomers thought both of these symptoms could be the result of NGC 4438 colliding with another galaxy. But if so, where was the second crash victim? To investigate, Jeffrey Kenney of Yale University and colleagues took a long exposure of a wide patch of sky around NGC 4438. The team found filaments of hydrogen gas connecting it to M86, a blob-shaped, or elliptical, galaxy about 400,000 light years away. The gas must have been ripped out from one or both of the galaxies by the collision. That comes as a surprise to some astronomers, who suspected that a different galaxy, much closer to NGC 4438, was involved. “Now it looks like we were fooled,” says William Keel of the University of Alabama in Tuscaloosa, who was not associated with the study. The collision may have sent shockwaves travelling at 1000 kilometres per second into M86, heating up its gas. Studying the crash could help astrophysicists work out why elliptical galaxies don’t form new stars. Ellipticals were once thought to have simply exhausted their gas supply. But x-ray observations revealed that these galaxies are full of gas, only it is too hot to condense down and form many new stars. Still, it is unclear what keeps the gas temperatures so high, around a million degrees Celsius. Some galaxies might be heated by active galactic nuclei (AGN), which blast out radiation as material is collected by a supermassive black hole. But many ellipticals don’t seem to contain powerful AGN. Instead, collisions or the steady collection of gas clouds might help heat up gas in these galaxies and so shut down star formation, Kenney says. Further study of the M86 crash might allow researchers to work out how much energy can be channelled into heat. “It’s the first case where there’s evidence a collision might have heated the gas in an elliptical galaxy,” Kenney told New Scientist. Journal reference: