Further to the previous posts regarding the nature and origin of the ring structures seen in M31, I note that D.L. Block et al,
in (2006),
Nature, Volume 443, p.832 provide some modeling that shows the physical plausibility of the two Mid-IR rings seen by Spitzer Telescope having an origin in an impact of M32 with the disk of M31. They interpret both rings as being density waves caused by an almost head-on collision of Andromeda Galaxy with M32, with the smaller galaxy plunging through the centre of M31.
I attach, here, the preprint of their paper in Nature:
M31___off centre rings caused by head on collision__(astroph 0610543]_[D.L. Block +__Nature_(200.pdf