RR Lyrae Variables in M31 and M33

TL;DR

RR Lyrae variables in M31 and M33 serve as key indicators of ancient stellar populations, revealing early formation and metallicity properties of these galaxies' halos and possibly disks, though more extensive surveys are needed.

Contribution

This paper reviews the presence and properties of RR Lyrae stars in M31 and M33, highlighting their significance in understanding galaxy formation and evolution.

Findings

RR Lyrae stars are present in the halos of M31 and M33.

The metallicities of these RR Lyrae stars are consistent with other halo tracers.

Current knowledge of their spatial distribution is limited, requiring further wide-field studies.

Abstract

The properties of RR Lyrae variables make them excellent probes of the formation and evolution of a stellar population. The mere presence of such stars necessitates an age greater than ~10 Gyr while their periods and amplitudes can be used to estimate the metal abundance of the cluster or galaxy in which they reside. These and other features of RR Lyraes have been used to study the properties of M31 and M33. Though these studies are generally in their infancy, we have established that M31 and M33 do indeed harbor RR Lyraes in their halos and probably also in their disks suggesting that these two components formed early in the history of M31 and M33. The mean metallicities of the halo RR Lyraes in these galaxies are consistent with those of other halo stellar population tracers such as the dwarf spheroidal satellites of M31 and the halo globular clusters in M33. Little is known about the spatial distribution of the RR Lyraes, especially in M33. This will require wide-field time-series studies with sufficient photometric depth to allow both the identification of RR Lyraes and robust period determination.