The Andromeda galaxy's most important merger about 2 billion years ago as M32's likely progenitor

Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies(1), uncertainty remains about M31s most important mergers. Previous studies focused individually on the giant stellar stream(2) or the impact of M32 on M31s disk(3,4), thereby suggesting many substantial satellite interactions(5). Yet models of M31s disk heating(6) and the similarity between the stellar populations of different tidal substructures in M31s outskirts(7) both suggested a single large merger. M31s stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites5 and provides information about its important mergers(8). Here we use cosmological models of galaxy formation(9,10) to show that M31s massive(11) and metal-rich12 stellar halo, containing intermediate-age stars(7), dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5 x 1010 solar masses, M-circle dot the third largest member of the Local Group) about 2 billion years (Gyr) ago. This single event explains many observations that were previously considered separately: M31s compact and metal-rich satellite M32(13) is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo(14) contains most of the merger debris, and the giant stellar stream(15) is likely to have been thrown out during the merger. This interaction may explain M31s global burst of star formation about 2 Gyr ago(16) in which approximately a fifth of its stars were formed. Moreover, M31s disk and bulge were already in place, suggesting that mergers of this magnitude need not dramatically affect galaxy structure.