PHAT. XIX. Formation history of M31 disk

We find that most of the star formation occurred prior to ~8Gyr ago, followed by a relatively quiescent period until ~4Gyr ago, a subsequent star formation episode about 2Gyr ago, and a return to relative quiescence.

There appears to be little, if any, structure visible for populations with ages older than 2Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa initial mass function from 0.1 to 100M_{sun}_, we find that the total amount of star formation over the past 14Gyr in the area over which we have fit models is 5x10^10^M_{sun}_. Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5+/-0.2)x10^11^M_{sun}_ of stars has formed in the M31 disk as a whole, (9+/-2)x10^10^M_{sun}_ of which has likely survived to the present after accounting for evolutionary effects.