A Global Star-forming Episode in M31 2-4 Gyr Ago
Bell, Eric F.; Monachesi, Antonela; Dolphin, Andrew E.; Williams, Benjamin F.; Gilbert, Karoline M.; Fouesneau, Morgan; Lang, Dustin; Weisz, Daniel R.; Dalcanton, Julianne J.; Skillman, Evan; Boyer, Martha; Lewis, Alexia R.
United States, Germany
Abstract
We have identified a major global enhancement of star formation in the inner M31 disk that occurred between 2-4 Gyr ago, producing ∼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.