  Although the proximity of the Andromeda galaxy (M31) offers an opportunity to
understand how mergers affect galaxies, uncertainty remains about M31's most
important mergers. Previous studies focused individually on the giant stellar
stream or the impact of M32 on M31's disk, thereby suggesting many substantial
satellite interactions. Yet models of M31's disk heating and the similarity
between the stellar populations of different tidal substructures in M31's
outskirts both suggested a single large merger. M31's stellar halo (its outer
low-surface-brightness regions) is built up from the tidal debris of satellites
and provides information about its important mergers. Here we use cosmological
models of galaxy formation to show that M31's massive and metal-rich stellar
halo, containing intermediate-age stars, dramatically narrows the range of
allowed interactions, requiring a single dominant merger with a large galaxy
(with stellar mass about 2.5 x 10^10 solar masses, the third largest member of
the Local Group) about 2 Gyr ago. This single event explains many observations
that were previously considered separately: M31's compact and metal-rich
satellite M32 is likely to be the stripped core of the disrupted galaxy, its
rotating inner stellar halo contains most of the merger debris, and the giant
stellar stream is likely to have been thrown out during the merger. This
interaction may explain M31's global burst of star formation about 2 Gyr ago in
which approximately a fifth of its stars were formed. Moreover, M31's disk and
bulge were already in place, suggesting that mergers of this magnitude need not
dramatically affect galaxy structure.
