Two major accretion epochs in M31 from two distinct populations of globular clusters

Large galaxies grow through the accumulation of dwarf galaxies. In principle it is possible to trace this growth history through the properties of a galaxy’s stellar halo. Previous investigations of M31 (Andromeda) showed that outside a radius of 25 kpc the population of halo globular clusters were rotating aligned with the stellar disk, as were more centrally located clusters. The M31 stellar halo also contains coherent substructures, along with a smooth component. Many of the globular clusters outside 25 kpc are associated with the most prominent substructures, while some are part of the smooth halo. Here we report on a new analysis of the kinematics of these globular clusters. We find two distinct populations rotating with perpendicular orientations. The rotation axis for the population associated with the smooth halo is aligned with that for the plane of dwarf galaxies that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, likely separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain