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We present deep Hubble Space Telescope Advanced Camera for Surveysobservations of the stellar populations in two fields lying at 20 and 23 kpcfrom the centre of M31 along the south-west semi-major axis. These data enablethe construction of colour-magnitude diagrams reaching the oldest main-sequenceturn-offs (~13 Gyr) which, when combined with another field at 25 kpc from ourprevious work, we use to derive the first precision constraints on thespatially-resolved star formation history of the M31 disc. The star formationrates exhibit temporal as well as field-to-field variations, but are generallyalways within a factor of two of their time average. There is no evidence ofinside-out growth over the radial range probed. We find a median age of ~7.5Gyr, indicating that roughly half of the stellar mass in the M31 outer disc wasformed before z ~ 1. We also find that the age-metallicity relations (AMRs) aresmoothly increasing from [Fe/H]~-0.4 to solar metallicity between 10 and 3 Gyrago, contrary to the flat AMR of the Milky Way disc at a similar number ofscale lengths. Our findings provide insight on the roles of stellar feedbackand radial migration in the formation and evolution of large disc galaxies.

The Spatially-Resolved Star Formation History of the M31 Outer Disc