The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31

We use Hubble Space Telescope optical imaging from the Panchromatic Hubble Andromeda Southern Treasury (PHAST) to measure the spatially resolved recent star formation history (SFH) across the southern disk of M31. We fit color-magnitude diagrams (CMDs) of over 6500 individual 0.01 kpc 2 regions to measure SFHs over the last ∼500 Myr. The resulting maps show coherent structure that traces the ringed morphology of the disk. We find a clear global decline in the recent star formation rate (SFR), with a pronounced drop in the last ∼40 Myr that is most evident in the region closest to M32. Combining PHAST and PHAT measurements, we now cover two-thirds of M31’s star-forming disk with homogeneous SFHs derived with the same technique, yielding the highest resolution spatially resolved SFHs of M31 measured with resolved stars. Inside the joint footprint, we measure a mean SFR of 0.445 ± 0.006 M ⊙ yr −1 over the last 100 Myr, and 0.285 ± 0.014 M ⊙ yr −1 over the last 20 Myr, implying a total disk SFR of ∼0.67 M ⊙ yr −1 and ∼0.43 M ⊙ yr −1 respectively, when extrapolated to the full star-forming disk. The observed decline is interpreted to be the late stages of a multi-Gyr wind-down from a previously more active state. Because recent star formation in M31 is concentrated primarily in the rings, the observed global decline is driven mainly by decreasing activity within those features. We also compare the CMD-based SFR surface densities to those inferred from far-ultraviolet (FUV)+24 μ m prescriptions and find that the FUV-based calibration underestimates the CMD-based 100 Myr average by a factor of ∼2.1. However, the PHAST SFHs produce a synthetic Galaxy Evolution Explorer FUV image that agrees well with observations, indicating that the CMD-derived SFHs provide an accurate description of recent star formation. The mismatch with the FUV+24 μ m estimates underscores that tracers implicitly averaged over ∼100 Myr are not reliable when the recent SFR is evolving.