Panchromatic Hubble Andromeda Treasury. 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$''{\times}$83$''$ (0.3$\times$1.4
kpc, deprojected) region of the PHAT survey outside of the innermost
6$'{\times}$12$'$ portion. We find that most of the star formation occurred
prior to $\sim$8 Gyr ago, followed by a relatively quiescent period until
$\sim$4 Gyr ago, a subsequent star formation episode about 2 Gyr ago and a
return to relative quiescence. There appears to be little, if any, structure
visible for populations with ages older than 2 Gyr, suggesting significant
mixing since that epoch. Finally, assuming a Kroupa IMF from 0.1$-$100
M$_{\odot}$, we find that the total amount of star formation over the past 14
Gyr in the area over which we have fit models is 5${\times}$10$^{10}$
M$_{\odot}$. Fitting the radial distribution of this star formation and
assuming azimuthal symmetry, (1.5$\pm$0.2)${\times}$10$^{11}$ M$_{\odot}$ of
stars have formed in the M31 disk as a whole, (9$\pm$2)${\times}$10$^{10}$
M$_{\odot}$ 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.
