Abstract
Abstract We measured [Fe/H] and [α/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red-giant-branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are α-enhanced, with [α/Fe] = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 and 12 kpc halo fields are relatively metal-poor, with [Fe/H] = −1.54 and −1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with [Fe/H] = −0.84 and −0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of a radial [α/Fe] gradient along the high surface brightness core of the GSS between ∼17 and 22 kpc. However, we found tentative suggestions of a negative radial [α/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [α/Fe] distribution of the metal-rich ([Fe/H] > −1.5), smooth inner stellar halo (rproj ≲ 26 kpc) is inconsistent with having formed from the disruption of a progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high α-enhancement provides support for an episode of rapid star formation in M31's disk possibly induced by a major merger.
Highlights
Stellar halos probe various stages of accretion history, as well as preserving signatures of in situ stellar formation (Font et al 2008, 2011; Zolotov et al 2009; Cooper et al 2010; Tissera et al 2013, 2014)
We present [α/Fe] and [Fe/H] measurements for three additional fields in the inner halo at rproj = 12 kpc, the Giant Stellar Stream (GSS) at rproj = 22 kpc, and outer disk of M31 at rproj = 26 kpc. These three fields, in addition to the smooth halo field of E19a, all overlap with Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) pointings with inferred color– magnitude diagram (CMD)-based star formation histories (SFHs; Brown et al 2006, 2007, 2009)
We referred to the stellar halo components in each field as belonging to the “inner halo” based on their projected radius, as opposed to any definition based on structural properties of the halo (Dorman et al 2013)
Summary
Stellar halos probe various stages of accretion history, as well as preserving signatures of in situ stellar formation (Font et al 2008, 2011; Zolotov et al 2009; Cooper et al 2010; Tissera et al 2013, 2014). The first work in this series (Escala et al 2019, hereafter E19a) applied a new technique of spectral synthesis of low-resolution (R ∼ 2500) spectroscopy to individual RGB stars in the smooth, metal-poor halo of M31 at rproj = 23 kpc. We present [α/Fe] and [Fe/H] measurements for three additional fields in the inner halo at rproj = 12 kpc, the GSS at rproj = 22 kpc, and outer disk of M31 at rproj = 26 kpc These three fields, in addition to the smooth halo field of E19a, all overlap with Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) pointings with inferred color– magnitude diagram (CMD)-based star formation histories (SFHs; Brown et al 2006, 2007, 2009).
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