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]
Highlights
1.1 RR Lyraes and their use in unveiling stellar populations The class of pulsating stars known as RR Lyrae variables are located at the intersection of the instability strip and the horizontal branch in the Hertzsprung Russell Diagram (Smith 1995)
Both color magnitude diagram (CMD) show a strong red giant branch (RGB), RC, asymptotic giant branch bump (≈ 1 mag above the RC), and RGB bump (≈ 0.5 mag below the RC). These features were first detected in the CMD presented by Monachesi et al (2011), constructed from an Advanced Camera for Surveys (ACS)/High Resolution Channel (HRC) field which is entirely overlapped by our observations
We have presented Hubble Space Telescope (HST)/ACS observations of two fields one that samples the stellar populations of M32 and another dubbed ‘Control’ that is dominated by the spheroid of M31
Summary
1.1 RR Lyraes and their use in unveiling stellar populations The class of pulsating stars known as RR Lyrae variables are located at the intersection of the instability strip and the horizontal branch in the Hertzsprung Russell Diagram (Smith 1995). Once the metallicities of the RR Lyraes are determined, their absolute magnitudes can be calculated using relations between [Fe/H] and the absolute magnitude of the RR Lyraes [MV (RR)], which is used to estimate the distance to their parent population, be it a star cluster or a galaxy. We have presented examples of how RR Lyrae variables can be powerful probes of the systems in which they reside - star clusters or among the field populations of galaxies. It is for this reason that studying them in Local Group galaxies like M31 and M32 provides valuable insights into the properties of these systems
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