The Star Formation Histories of the M31 and M33 Spheroids

TL;DR

This paper reviews the star formation histories of the spheroids in the Local Group's spiral galaxies M31 and M33, highlighting their different evolutionary paths and stellar populations based on observational data.

Contribution

It provides a comparative analysis of the star formation histories and stellar populations in the spheroids of M31 and M33, based on recent observational constraints.

Findings

M33 lacks a traditional bulge and has a nuclear region with diverse stellar ages.

M31's halo contains both ancient and younger, metal-rich stars due to merger activity.

The star formation history of M33's halo remains poorly constrained.

Abstract

I review the observational constraints on the star formation histories in the spheroids of M33 and M31, the other two spiral galaxies in the Local Group. M33 does not possess a traditional bulge; instead, it has a small nuclear region hosting stars with a wide range of ages. The star formation history of the M33 halo is poorly constrained, but composite spectra of its halo globular clusters imply a wide age spread of 5 - 7 years, while the presence of RR Lyrae stars in the halo implies at least some of the population is ancient. Although it is possible to obtain the detailed star formation history of the M33 halo via deep photometry, this has not been done to date. M31 hosts a traditional bulge that is apparently dominated by stars older than 10 Gyr. Deep photometry of the M31 halo demonstrates that it hosts both a population of ancient metal-poor stars and a significant population extending to younger ages and high metallicity, apparently due to its active merger history.