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We propose to create a legacy map of the southern half of M31 in the optical and near ultraviolet (NUV). The Panchromatic Hubble Andromeda Southern Treasury (PHAST) will add virgul100 million stars to M31.s stellar photometry archive and cover regions of M31 that are structurally unique and more sensitive to the merger history than the northern disk mapped in the Panchromatic Hubble Andromeda Treasury (PHAT). These newly mapped regions include the intersections with M32 and the giant southern stream comma the split portion of the star forming ring comma and the southern bar. By providing age comma metallicity comma stellar mass comma and follow-up kinematics comma the resolved stellar photometry of these regions will be used for detailed comparisons to simulations comma including star formation history and population asymmetries as well as the orbits of M32 and the giant southern stream comma allowing us to distinguish between competing merger scenarios. Thus comma these measurements tightly constrain models of M31.s merger history and disk evolution. Furthermore comma the legacy value for such observations is undeniable M31 has been and will continue to be a centerpiece target for many NASA and ESA missions comma as well as for ground-based time-domain observations that could find extremely valuable events that require HST resolution for identifying precursors. These current and future data sets cover the entire galaxy comma and thus comma our coverage will increase the value of all past and future M31 observations. PHAST will complete a lasting baseline data set for this high-demand target that no other current or planned facility will be able to produce.