  The unusual morphology of the Andromeda Spiral (Messier 31, the closest
spiral galaxy to the Milky Way) has long been an enigma. Although regarded for
decades as showing little evidence of a violent history, M~31 has a well-known
outer ring of star formation at a radius of 10 kpc whose center is offset from
the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at
both optical and radio wavelengths. The halo contains numerous loops and
ripples. Here we report the discovery, based on analysis of previously-obtained
data, of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and
offset by ~0.5kpc from the center of the galaxy. The two rings appear to be
density waves propagating in the disk. Numerical simulations offer a completely
new interpretation for the morphology of M31: both rings result from a
companion galaxy plunging head-on through the center of the disk of M31. The
most likely interloper is M32. Head-on collisions between galaxies are rare,
but it appears nonetheless that one took place 210 million years ago in our
Local Group of galaxies.
