RR Lyrae Variables in the Halo of M33

TL;DR

This study confirms the presence of RR Lyrae stars in M33's halo and possibly its disk, indicating early formation of these components and providing insights into the galaxy's ancient stellar populations.

Contribution

First detection of RR Lyrae variables in M33's halo and potential disk, linking their properties to the galaxy's formation history.

Findings

RR Lyrae stars are present in M33's halo and possibly disk.

Halo RR Lyrae metallicity is about [Fe/H]~-1.3.

Early formation of halo and disk components suggested.

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 Local Group late-type spiral galaxy M33. Though these studies are generally in their infancy, we have established that M33 does indeed harbor RR Lyraes in its halo and probably also in its disk suggesting that these two components formed early in the history of M33. The mean metallicity of the halo RR Lyraes is consistent with that of the halo globular clusters in M33 at [Fe/H]~-1.3. Little is known about the spatial distribution of the RR Lyraes; this will require wide-field time-series studies with sufficient photometric depth to allow both the identification of RR Lyraes and robust period determination.