The Recent Stellar Archeology of M31 - The Nearest Red Disk Galaxy

TL;DR

This study investigates the recent star formation history of M31, revealing increased activity in the inner disk over the past 10 million years, and suggests that certain structures are remnants of past interactions, with implications for galaxy evolution.

Contribution

It provides new insights into the spatial and temporal variations of star formation in M31, highlighting the age and dynamics of gas rings and asymmetries in stellar distributions.

Findings

Inner disk star formation rate increased 2-3 times in last 10 Myr.

Gas rings are at least 100 Myr old and have unique stellar velocity properties.

Asymmetry in red star distribution suggests past interactions with a companion galaxy.

Abstract

We examine the star-forming history (SFH) of the M31 disk during the past few hundred Myr. The luminosity functions (LFs) of main sequence stars at distances R_GC > 21 kpc (i.e. > 4 disk scale lengths) are matched by models that assume a constant star formation rate (SFR). However, at smaller R_GC the LFs suggest that during the past ~10 Myr the SFR was 2 - 3 times higher than during the preceding ~100 Myr. The rings of cool gas that harbor a significant fraction of the current star-forming activity are traced by stars with ages ~100 Myr, indicating that (1) these structures have ages of at least 100 Myr, and (2) stars in these structures do not follow the same relation between age and random velocity as their counterparts throughout the disks of other spiral galaxies, probably due to the inherently narrow orbital angular momentum distribution of the giant molecular clouds in these structures. The distribution of evolved red stars is not azimuthally symmetric, in the sense that their projected density along the north east segment of the major axis is roughly twice that on the opposite side of the galaxy. The north east arm of the major axis thus appears to be a fossil star-forming area that dates to intermediate epochs. Such a structure may be the consequence of interactions with a companion galaxy.