Abstract

Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies¹, uncertainty remains about M31's most important mergers. Previous studies focused individually on the giant stellar stream² or the impact of M32 on M31's disk^3,4, thereby suggesting many substantial satellite interactions⁵. Yet models of M31's disk heating⁶ and the similarity between the stellar populations of different tidal substructures in M31's outskirts⁷ both suggested a single large merger. M31's stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites⁵ and provides information about its important mergers⁸. Here we use cosmological models of galaxy formation^9,10 to show that M31's massive¹¹ and metal-rich¹² stellar halo, containing intermediate-age stars⁷, dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5 × 10¹⁰ solar masses, M_⊙ 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: M31's compact and metal-rich satellite M32¹³ is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo¹⁴ contains most of the merger debris, and the giant stellar stream¹⁵ is likely to have been thrown out during the merger. This interaction may explain M31's global burst of star formation about 2 Gyr ago¹⁶ 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.