PHAT. XIX. the Ancient Star Formation History of the M31 Disk

We map the star formation history across M31 by fitting stellar evolution models to color-magnitude diagrams of each 83×83 (0.3?×1.4 kpc, deprojected) region of the Panchromatic Hubble Andromeda Treasury (PHAT) survey outside of the innermost 6 12 portion. We find that most of the star formation occurred prior to ∼8 Gyr ago, followed by a relatively quiescent period until ∼4 Gyr ago, a subsequent star formation episode about 2 Gyr ago, and a return to relative quiescence. There appears to be little, if any, structure visible for populations with ages older than 2 Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa initial mass function from 0.1 to 100 Me, we find that the total amount of star formation over the past 14 Gyr in the area over which we have fit models is 5 × 1010 Me. Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5±0.2)?× 1011 Me of stars has formed in the M31 disk as a whole, (9±2)×1010Me of which has likely survived to the present after accounting for evolutionary effects. This mass is about one-fifth of the total dynamical mass of M31.