A Global Star Forming Episode in M31 2-4 Gyr Ago

TL;DR

This paper identifies a major, galaxy-wide star formation episode in M31 2-4 billion years ago, contributing to most of its recent stellar mass, and discusses its possible causes and implications.

Contribution

It reveals a previously unconfirmed, massive, and global star formation event in M31's history, based on detailed star color-magnitude diagram modeling.

Findings

Approximately 60% of M31's stars formed during this episode.

The star formation rate declined significantly after the burst.

The event was both massive and galaxy-wide, affecting inner and outer disks.

Abstract

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.