Friday Catastrophysics: Battered Galaxies part 1

Our cosmic neighborhood is not as quiet and suburban as you might imagine. New research using the Spitzer Space Telescope suggests that the nearest giant spiral galaxy (i.e. similar in type and size to our own Milky Way galaxy) took a direct hit from another galaxy, triggering an ripple of star formation that has now propagated 30 000 light years outward.

The image to the left is an optical image of M31 and its satellite galaxies: M32 (the bright white dot at center left) and M110 (the fuzzy blob toward the bottom right hand side), taken by Robert Gendler.

Block et al (2006, Nature; also see astro-ph/0610543) base this interpretation on mid-infrared imaging at a wavelength of 8 micron, which is sensitive to warm dust (and starlight, but they can remove the starlight using 3 micron imaging which is only sensitive to starlight). The resulting image of M31 is shown in red here, along with a the optical image rotated and scaled (by eye) to match the orientation of the Spitzer image.


Although there is something of a weak distorted spiral arm pattern to M31, the 8 micron image shows closed ring-like structures. The outer red ring is a region of hot dust, heated by the young massive stars that have recently formed there. This star-forming ring has been know about for some time, but it is the inner dust ring that is the new discovery.

Star-forming rings can be produced by two mechanisms. The more standard one is for a stellar bar to create inner and outer rings, but M31's bar is relatively weak and not as large as the 10-kiloparsec radius (30 000 light year) outer ring, nor is it aligned with the inner dust ring. The other way to get a large-radius ring of gas and dust in a spiral galaxy is for another galaxy to pass through the disk on a polar orbit. Here the culprit is suspected to be M32, the smaller of the dwarf galaxies visible in the image of M31.

This form of galaxy collision has been seen before, and in the more extreme cases (where the colliding objects are closer in size and mass) the disruption of the impacted galaxy can be severe, as can be seen below in the image of the Cartwheel Galaxy. These objects are typically called ring galaxies. Another example worth a look at is NGC 4650A.



An an aside, Astroprof had a blog post about our (the Milky Way) not-so-imminent collision with M31 just the day before the Block et al results were released.