A broad region of gas glowing only at x-ray wavelengths marks the crumple zone where a dwarf galaxy is slamming into a much larger mass of stars, a recent study suggests. The larger galaxy, dubbed NGC 1232, lies about 60 million light-years away from Earth in the constellation Eridanus. In visible wavelengths, it looks like a normal spiral galaxy. But in a visible light/x-ray composite image released today, a purplish, comet-shaped glow across the galaxy’s face depicts a region—estimated to be almost 24,000 light-years across—where gas is being heated to temperatures of about 5.8 million°C. The glowing gas cloud may be up to 3 million times as massive as our sun, and it is much too voluminous to represent a normal star-forming region, which is typically less than 300 light-years across and fits within one of a galaxy’s spiral arms. The cloud’s large size, as well as the lack of any distinct features at optical or infrared wavelengths, also argues against the glow being the remnants of a supernova explosion. Instead, the researchers have proposed, the x-ray glow betrays the presence of a shock wave somewhat akin to a sonic boom, triggered by a dwarf galaxy about 1/10,000th the size of NGC 1232 or smaller slamming into the diffuse halo of gas surrounding its larger cousin. Although galactic collisions have been seen many times before, this is the first to be seen only at x-ray wavelengths, the researchers say. And because NGC 1232 looks normal at visible wavelengths, the team suggests that collisions with dwarf galaxies—which in many cases are too small to cause disturbances in the motions of stars within the larger galaxy—are probably rare but may be more common than recognized. One region glowing very intensely in x-ray wavelengths (depicted by arrow) appears to be located at the leading edge of the cometlike glow, so it may represent an area of prolific star formation triggered by the shock wave resulting from the galactic collision.
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