A Global Star Forming Episode in M31 2-4 Gyr Ago
  We have identified a major global enhancement of star formation in the inner
M31 disk that occurred between 2-4 Gyr ago, producing $\sim$60% of the stellar
mass formed in the past 5 Gyr. The presence of this episode in the inner disk
was discovered by modeling the optical resolved star color-magnitude diagrams
of low extinction regions in the main disk of M31 (3$<$R$<$20 kpc) as part of
the Panchromatic Hubble Andromeda Treasury. This measurement confirms and
extends recent measurements of a widespread star formation enhancement of
similar age in the outer disk, suggesting that this burst was both massive and
global. Following the galaxy-wide burst, the star formation rate of M31 has
significantly declined. We briefly discuss possible causes for these features
of the M31 evolutionary history, including interactions with M32, M33 and/or a
merger.
