Abstract
Abstract Current instruments and spectral analysis programs are now able to decompose the integrated spectrum of a stellar system into distributions of ages and metallicities. The reliability of these methods has rarely been tested on nearby systems with resolved stellar ages and metallicities. Here we derive the age–metallicity distribution of M54, the nucleus of the Sagittarius dwarf spheroidal galaxy, from its integrated Multi-Unit Spectroscopic Explorer (MUSE) spectrum. We find a dominant old (8–14 Gyr), metal-poor (−1.5 dex) component and a young (1 Gyr), metal-rich (+0.25 dex) component—consistent with the complex stellar populations measured from individual stars in the same MUSE data set. There is excellent agreement between the (mass-weighted) average age and metallicity of the resolved and integrated analyses. Differences are only 3% in age and 0.2 dex metallicity. By co-adding individual stars to create M54's integrated spectrum, we show that the recovered age–metallicity distribution is insensitive to the magnitude limit of the stars or the contribution of blue horizontal branch stars—even when including additional blue wavelength coverage from the WiFeS Atlas of Galactic Globular cluster Spectra survey. However, we find that the brightest stars can induce the spurious recovery of an old (>8 Gyr), metal-rich (+0.25 dex) stellar population, which is otherwise not expected from our understanding of chemical enrichment in M54. The overall derived stellar mass-to-light ratio of M54 is M/L V = 1.46 with a scatter of 0.22 across the field of view, which we attribute to the stochastic contribution of a young, metal-rich component. These findings provide strong evidence that complex stellar population distributions can be reliably recovered from integrated spectra of extragalactic systems.
Highlights
Analyzing ages and metallicities or other chemical abundances of any stellar systems give us insight into their assembly history
We find a dominant old (8 − 14 Gyr), metal-poor (-1.5 dex) and a young (1 Gyr), metal-rich (+0.25 dex) component - consistent with the complex stellar populations measured from individual stars in the same MUSE data set
This paper is organized as follows: in Section 2 we present the three different integral field unit (IFU) data sets analyzed in this work and briefly describe the analysis of the resolved stars from Alfaro-Cuello et al (2019); in Section 3 we describe our analysis method of deriving age-metallicity distributions from integrated spectra; in Section 4 we show the results of this technique in dependence of different integrated spectra of M 54; in Section 5 we compare our integrated analysis method with the resolved star analysis; in Section 6 we discuss our results and in Section 7 we give our conclusions of this comparison exercise
Summary
Analyzing ages and metallicities or other chemical abundances of any stellar systems give us insight into their assembly history. Galaxies and other stellar objects (e.g. nuclear star clusters (NSCs), see Neumayer et al 2020, for a review) are assembled by a combination of in-situ secular processes such as star formation, as well as ex-situ accretion of other systems. The most detailed studies of stellar populations in NSCs are restricted to the nearby ones in the center of the Milky Way (MW: Do et al 2009, 2013; Feldmeier et al 2014; FeldmeierKrause et al 2015, 2017a,b) and the Sagittarius dwarf spheroidal galaxy (M 54: Siegel et al 2007; Bellazzini et al 2008; Alfaro-Cuello et al 2019, 2020))
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