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 using the properties of a galaxy's
stellar halo. Previous investigations of the galaxy M31 (Andromeda) have shown
that outside a radius of 25 kpc the population of halo globular clusters is
rotating in alignment with the stellar disk, as are more centrally located
clusters. The M31 halo also contains coherent stellar substructures, along with
a smoothly distributed stellar component. Many of the globular clusters outside
25 kpc are associated with the most prominent substructures, while others are
part of the smooth halo. Here we report a new analysis of the kinematics of
these globular clusters. We find that the two distinct populations are rotating
with perpendicular orientations. The rotation axis for the population
associated with the smooth halo is aligned with the rotation axis 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.
