The Nature and Origin of Substructure in the Outskirts of M31 -- II.
  Detailed Star Formation Histories
  While wide-field surveys of M31 have revealed much substructure at large
radii, understanding the nature and origin of this material is not
straightforward from morphology alone. Using deep HST/ACS data, we have derived
further constraints in the form of quantitative star formation histories (SFHs)
for 14 inner halo fields which sample diverse substructures. In agreement with
our previous analysis of colour-magnitude diagram morphologies, we find the
resultant behaviours can be broadly separated into two categories. The SFHs of
'disc-like' fields indicate that most of their mass has formed since z~1, with
one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields
to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed
within the last 5 Gyr. These fields are also characterised by an
age--metallicity relation showing rapid chemical enrichment to solar
metallicity by z=1, suggestive of an early-type progenitor. We confirm a
significant burst of star formation 2 Gyr ago, discovered in our previous work,
in all the fields studied here. The presence of these young stars in our most
remote fields suggests that they have not formed in situ but have been
kicked-out from through disc heating in the recent past.
