Abstract
Abstract Measurements of [Fe/H] and [α/Fe] can probe the minor merging history of a galaxy, providing a direct way to test the hierarchical assembly paradigm. While measurements of [α/Fe] have been made in the stellar halo of the Milky Way (MW), little is known about detailed chemical abundances in the stellar halo of M31. To make progress with existing telescopes, we apply spectral synthesis to low-resolution DEIMOS spectroscopy (R ∼ 2500 at 7000 Å) across a wide spectral range (4500 Å < λ < 9100 Å). By applying our technique to low-resolution spectra of 170 giant stars in five MW globular clusters, we demonstrate that our technique reproduces previous measurements from higher resolution spectroscopy. Based on the intrinsic dispersion in [Fe/H] and [α/Fe] of individual stars in our combined cluster sample, we estimate systematic uncertainties of ∼0.11 dex and ∼0.09 dex in [Fe/H] and [α/Fe], respectively. We apply our method to deep, low-resolution spectra of 11 red giant branch stars in the smooth halo of M31, resulting in higher signal-to-noise ratios per spectral resolution element compared to DEIMOS medium-resolution spectroscopy, given the same exposure time and conditions. We find [α/Fe] = 0.49 ± 0.29 dex and [Fe/H] = −1.59 ± 0.56 dex for our sample. This implies that—much like the MW—the smooth halo field of M31 is likely composed of disrupted dwarf galaxies with truncated star formation histories that were accreted early in the halo’s formation.
Highlights
Stellar chemical abundances are a key component in determining the origins of stellar halos of Milky Way (MW)like galaxies, providing insight into the formation of galaxyscale structure
Given the variety in approaches of high-resolution spectroscopy (HRS) studies of the MW globular clusters (GCs), MW dwarf spheroidal (dSph), and MW halo stars listed in Table 1, we provide a summary of the stellar parameter determination and abundance analysis in each case
MW dSphs: For Canes Venatici I (CVnI) and Ursa Minor (UMi), we find a single star in common between each of our DEIMOS slitmasks and the HRS literature (Shetrone et al 2001; François et al 2016)
Summary
Stellar chemical abundances are a key component in determining the origins of stellar halos of Milky Way (MW)like galaxies, providing insight into the formation of galaxyscale structure. It is possible to obtain spectroscopic metallicity measurements of M31 RGB stars from medium-resolution spectra (MRS; R ∼ 6000) using spectral synthesis (e.g., Kirby et al 2008a) This method leverages the entire spectrum’s metallicity information simultaneously, enabling measurements of abundances from relatively low-S/N spectra. Kirby et al (2008b, 2010, 2013) successfully measured [Fe/H] and [α/Fe] in MW globular clusters (GCs), MW dwarf spheroidal (dSph) satellite galaxies, and Local Group dwarf irregular galaxies, showing that abundances can be measured to a precision of ∼0.2 dex from spectra with S/N ∼ 15 Å−1 It was only in 2014 that spectral synthesis was applied to individual RGB stars in the M31 system for the first time (Vargas et al 2014a, 2014b).
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