Abstract details

The ages of stellar populations in the far outer disks of spiral galaxies can provide checks on models of hierarchical galaxy formation in which spiral disks form after a redshift of ~1 and in which disks are expected to form inside-out. We present deep (~8 mags below the RGB tip) HST/ACS colour magnitude diagrams of two fields in the warp and outer disk of M31. By modelling the CMDs with synthetic stellar populations we can place constraints on the star formation histories in these two fields.