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

Abstract

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 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 M ⊙ , 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 × 10 10   M ⊙ . Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5 ± 0.2) × 10 11   M ⊙ of stars has formed in the M31 disk as a whole, (9 ± 2) × 10 10   M ⊙ 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.