The Spatially-Resolved Star Formation History of the M31 Outer Disc
  We present deep Hubble Space Telescope Advanced Camera for Surveys
observations of the stellar populations in two fields lying at 20 and 23 kpc
from the centre of M31 along the south-west semi-major axis. These data enable
the construction of colour-magnitude diagrams reaching the oldest main-sequence
turn-offs (~13 Gyr) which, when combined with another field at 25 kpc from our
previous work, we use to derive the first precision constraints on the
spatially-resolved star formation history of the M31 disc. The star formation
rates exhibit temporal as well as field-to-field variations, but are generally
always within a factor of two of their time average. There is no evidence of
inside-out growth over the radial range probed. We find a median age of ~7.5
Gyr, indicating that roughly half of the stellar mass in the M31 outer disc was
formed before z ~ 1. We also find that the age-metallicity relations (AMRs) are
smoothly increasing from [Fe/H]~-0.4 to solar metallicity between 10 and 3 Gyr
ago, contrary to the flat AMR of the Milky Way disc at a similar number of
scale lengths. Our findings provide insight on the roles of stellar feedback
and radial migration in the formation and evolution of large disc galaxies.
