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We map the star formation history across M31 by fitting stellar evolutionmodels to color-magnitude diagrams of each 83$''{\times}$83$''$ (0.3$\times$1.4kpc, deprojected) region of the PHAT survey outside of the innermost6$'{\times}$12$'$ portion. We find that most of the star formation occurredprior to $\sim$8 Gyr ago, followed by a relatively quiescent period until$\sim$4 Gyr ago, a subsequent star formation episode about 2 Gyr ago and areturn to relative quiescence. There appears to be little, if any, structurevisible for populations with ages older than 2 Gyr, suggesting significantmixing since that epoch. Finally, assuming a Kroupa IMF from 0.1$-$100M$_{\odot}$, we find that the total amount of star formation over the past 14Gyr in the area over which we have fit models is 5${\times}$10$^{10}$M$_{\odot}$. Fitting the radial distribution of this star formation andassuming azimuthal symmetry, (1.5$\pm$0.2)${\times}$10$^{11}$ M$_{\odot}$ ofstars 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 forevolutionary effects. This mass is about one fifth of the total dynamical massof M31.

Panchromatic Hubble Andromeda Treasury. XIX. The Ancient Star Formation History of the M31 Disk