An analysis of the composite stellar population in M32

TL;DR

This study uses high-quality spectral data to analyze the stellar populations of M32, revealing a coexistence of ancient and intermediate-age stars with varying contributions across different regions.

Contribution

It provides a detailed comparison of integrated light spectroscopy results with resolved stellar population data, highlighting the spatial variation of stellar populations in M32.

Findings

Ancient and intermediate-age populations coexist in M32.

The intermediate population is more prominent at the nucleus.

Spectroscopic metallicity distribution shows a broader range with low-metallicity component.

Abstract

We obtained long-slit spectra of high signal-to-noise ratio of the galaxy M32 with the GMOS spectrograph at the GEMINI North telescope. We analysed the integrated spectra by means of full spectral fitting in order to extract the mixture of stellar populations that best represents its composite nature. Three different galactic radii were analysed, from the nuclear region out to 2 arcmin from the centre. This allows us to compare, for the first time, the results of integrated light spectroscopy with those of resolved colour-magnitude diagrams from the literature. As our main result, we propose that an ancient and an intermediate-age population coexist in M32, and that the balance between these two populations change between the nucleus and outside 1 effective radius in the sense that the contribution from the intermediate population is larger at the nuclear region. We retrieve a smaller signal of a young population at all radii whose origin is unclear and may be a contamination from horizontal-branch stars, such as the ones identified by Brown et al. in the nuclear region. We compare our metallicity distribution function for a region 1 to 2 arcmin from the centre to the one obtained with photometric data by Grillmair et al. Both distributions are broad, but our spectroscopically derived distribution has a significant component with $[Z/Z_{\sun}] \leq -1$, which is not found by Grillmair et al.