Abstract details

Although it is now well-established that M31 possesses a wealth of stellar substructure at large radius, the nature of this substructure remains unclear. Does it represent the residue of one or more accreted dwarf galaxies? Or simply the perturbed and warped regions of the outer disk? We present deep HST/ACS colour-magnitude diagrams of seven distinct regions of stellar substructure in M31, as well as one clean field along the minor axis, which reach ~7 magnitudes below the red giant branch tip. We use these data to place detailed constraints on the star formation history of these overdensities via synthetic CMD-modelling and discuss how the nature of the substructure compares to that expected in models where the stellar halo assembles as a direct result of satellite accretion.