It would have been quite a collision. Around 210 million years ago, galaxy M31 — better known as Andromeda — and its smaller neighbour M32 met almost head-on, leaving the battered structure of M31 seen today. This striking proposal is made by D. L. Block et al. on page 832 of this issue ( Nature 443, 832–834; 2006).
The story is summed up by the two snapshots, each roughly 120,000 light years across,included here. They show the results of Block and colleagues' numerical simulations, with (top) a serene M31 some 35 million years before the collision, and (below) the dishevelled M31 of today.
The simulations offer telling support for the authors' scheme of events, which hinges on two dust rings in the galaxy's disk (see Fig. 1 on page 832). One, at a radius of about 33,000 light years, is well known; the other, much closer in, was identified only recently with NASA's Spitzer Space Telescope. The two rings emerge from the simulations — precise in both position and orientation — as a consequence of a head-on galaxy–galaxy impact, with M32 being the likely culprit.
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Lincoln, T. Andromeda's troubled past. Nature 443, 762 (2006). https://doi.org/10.1038/443762b