An analysis of the composite stellar population in M32^★

Abstract

We obtained long‐slit spectra of high signal‐to‐noise ratio of the galaxy M32 with the Gemini Multi‐Object 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 a main result we propose that an ancient and an intermediate‐age population co‐exist in M32, and that the balance between these two populations change between the nucleus and outside one effective radius (1reff) 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⊙]≤−1, which is not found by Grillmair et al.

Fake Star Formation Bursts: Blue Horizontal Branch Stars Masquerade as Young Massive Stars in Optical Integrated Light Spectroscopy

“…Even though we do not constrain well the extent of the "confusion zone" in the high Z regime, the young component is stronger than the fake bursts we found: it accounts for 20% of the light (in a similar wavelength range) while we found maximum spurious contributions of 10%-12%. Hence, our result supports the claims of Coelho et al (2009), mainly that there has been a recent burst of star formation in the center of M32, while the young component seen in the outskirts is also compatible with being an artifact due to the blue HB morphology of the system.…”

“…The rightmost panel of Figure 6 of Coelho et al 2009 shows that except for the nucleus, a large fraction (about ≈50 %) of the light of M32 is produced by a [Fe/H] < −1.2 old stellar population. Our study shows that in this regime, a spurious young burst accounting for ≈5% is indeed likely to appear.…”

“…The effect on star formation history (hereafter SFH) reconstructions could be even more dramatic: do the missing hot stars show up as an additional fake burst as suggested in Coelho et al (2009), or do they simply shift the whole age distribution to younger ages? At the moment, no studies have been published specifically on the effect of HB morphology on the significance of SFH reconstructions from full spectrum fitting in the optical.…”

Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities

“…In addition, while Fan & de Grijs (2012) Photometric, spectral indices or spectral fitting methods differ not only in their technical details, but also their underlying stellar population models differ in their choice of evolutionary tracks, the libraries of stellar spectra and in their implementation details, such as interpolations. Coelho et al (2009), for example, made an analysis of the stellar population in M 32 and has shown that using the same method with different SSP models result in different age and metallicity distributions; they conclude that choosing different SSP models from the literature might yield different results of age and metallicity at the quantitative level, even though a qualitative agreement is met. Dias et al (2010) also found evidence of this dependence of the results on the choice of SSP models when studying integrated spectra of GCs in the Small Magellanic Cloud.…”

The Stellar Populations of M32: Resolving the nearest elliptical with HST ACS/HRC

“…M32 appears to have had one or more relatively recent episodes of star formation (within the last 3 Gyr: e.g., O'Connell 1980;Rose 1985;Trager et al 2000;Coelho et al 2009). O'Connell (1980) concluded that models fail to reproduce M32 with a single old-age and solar-metallicity population.…”