Abstract
Abstract We analyze existing measurements of [Fe/H] and [α/Fe] for individual red giant branch (RGB) stars in the Giant Stellar Stream (GSS) of M31 to determine whether spatial abundance gradients are present. These measurements were obtained from low- (R ∼ 3000) and moderate- (R ∼ 6000) resolution Keck/DEIMOS spectroscopy using spectral synthesis techniques as part of the Elemental Abundances in M31 survey. From a sample of 62 RGB stars spanning the GSS at 17, 22, and 33 projected kpc, we measure a [Fe/H] gradient of −0.018 ± 0.003 dex kpc−1 and negligible [α/Fe] gradient with M31-centric radius. We investigate GSS abundance patterns in the outer halo using additional [Fe/H] and [α/Fe] measurements for six RGB stars located along the stream at 45 and 58 projected kpc. These abundances provide tentative evidence that the trends in [Fe/H] and [α/Fe] beyond 40 kpc in the GSS are consistent with those within 33 kpc. We also compare the GSS abundances to 65 RGB stars located along the possibly related Southeast (SE) shelf substructure at 12 and 18 projected kpc. The abundances of the GSS and SE shelf are consistent, supporting a common origin hypothesis, although this interpretation may be complicated by the presence of [Fe/H] gradients in the GSS. We discuss the abundance patterns in the context of photometric studies from the literature and explore implications for the properties of the GSS progenitor, suggesting that the high 〈[α/Fe]〉 of the GSS (+0.40 ± 0.05 dex) favors a major merger scenario for its formation.
Highlights
Stellar streams originate from the ongoing tidal disruption of accreted galaxies and globular clusters, providing an instantaneous view of the hierarchical formation of the host galaxy (e.g., Freeman & Bland-Hawthorn 2002; Bullock & Johnston 2005; Helmi 2020)
We present a comprehensive analysis of spatial [Fe/H] and [α/Fe] gradients in the Giant Stellar Stream (GSS) and likely associated substructures using spectral synthesis–based abundance measurements from the Elemental Abundances in M31 survey (Escala et al 2019, 2020a, 2020b; Gilbert et al 2019, 2020; Kirby et al 2020; Wojno et al 2020) with the aim of providing further constraints for GSS formation models
In contrast to previous work by Escala et al (2020b) using a nearly identical sample, we focused our analysis on M31 red giant branch (RGB) stars with a high probability of belonging to kinematically identifiable substructure based on their heliocentric radial velocities
Summary
Stellar streams originate from the ongoing tidal disruption of accreted galaxies and globular clusters, providing an instantaneous view of the hierarchical formation of the host galaxy (e.g., Freeman & Bland-Hawthorn 2002; Bullock & Johnston 2005; Helmi 2020). In comparison to the phase-mixed component of M31ʼs stellar halo, photometric and spectroscopic studies of the GSS’s resolved stellar populations have revealed that it is more metal-rich and kinematically colder and possesses more dominant intermediate-age stellar populations (e.g., Guhathakurta et al 2006; Brown et al 2006; Kalirai et al 2006; Ibata et al 2007; Gilbert et al 2009; Tanaka et al 2010; Ibata et al 2014) Based on these properties, the GSS was inferred to originate from the recent ( 1 Gyr) disruption of a distinct satellite progenitor on a highly radial orbit with a lower stellar mass limit of ∼ 108 Me (Ibata et al 2004; Fardal et al 2006; Font et al 2006b). We provide a brief summary of the spectroscopic observations and abundance measurements below and refer the reader to Gilbert et al (2019, 2020) and Escala et al (2019, 2020a, 2020b) for further details
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