An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars

TL;DR

This study uses RR Lyrae stars to identify and analyze ancient, metal-poor stellar populations in M32 and M31, revealing insights into galaxy formation, halo composition, and satellite accretion processes.

Contribution

It provides the first evidence of an ancient metal-poor population in M32 and identifies two distinct RR Lyrae populations in M31, indicating complex formation histories.

Findings

RR Lyrae stars confirm ancient population in M32

Two RR Lyrae populations in M31 with different metallicities

Evidence of dwarf satellite accretion influencing M31 halo

Abstract

We present time-series photometry of two fields near M32 using archival observations from ACS/WFC onboard HST. One field is centered about 2 arcmin from M32 while the other is located 15 arcmin to the southeast of M31. We identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318 are c-type. In the field near M32, we find a radial gradient in the density of RR Lyraes relative to the center of M32. This gradient is consistent with the surface brightness profile of M32 suggesting that a significant number of the RR Lyraes in this region belong to M32. This provides further confirmation that M32 contains an ancient stellar population formed around the same time as the oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a mean metal abundance of [Fe/H] ~ -1.42 +/- 0.02, which is ~15 times lower than the metal abundance of the overall M32 stellar population. Moreover, the abundance of RR Lyrae stars normalized to the luminosity of M32 in the field analyzed further indicates that the ancient metal-poor population in M32 represents only a very minor component of this galaxy, consistent with the 1% to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other field, we find unprecedented evidence for two populations of RR Lyraes in M31 as shown by two distinct sequences among the ab-type variables in the Bailey Diagram. When interpreted in terms of metal abundance, one population exhibits a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible interpretation of this result is that the more metal-rich population represents the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf satellite galaxy orbiting M31. If true, this represents a further indication that the formation of the M31 spheroid has been significantly influenced by the merger and accretion of dwarf galaxy satellites. [abridged]