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.