The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories

TL;DR

This study uses deep HST data to derive star formation histories of 14 M31 halo fields, revealing two main substructure types with distinct formation timelines and chemical enrichment patterns, including recent star formation bursts.

Contribution

It provides detailed quantitative star formation histories for M31's halo substructures, distinguishing between 'disc-like' and 'stream-like' origins with implications for galaxy formation.

Findings

'Disc-like' fields formed most of their stars since z~1.

'Stream-like' fields are older with rapid early chemical enrichment.

A significant star formation burst occurred 2 Gyr ago across fields.

Abstract

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep HST/ACS data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour-magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed since z~1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed within the last 5 Gyr. These fields are also characterised by an age--metallicity relation showing rapid chemical enrichment to solar metallicity by z=1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from through disc heating in the recent past.