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Elemental Abundances in M31: Properties of the Inner Stellar Halo

Escala, Ivanna and Kirby, Evan N. and Gilbert, Karoline M. and Wojno, Jennifer and Cunningham, Emily C. and Guhathakurta, Puragra (2020) Elemental Abundances in M31: Properties of the Inner Stellar Halo. Astrophysical Journal, 902 (1). Art. No. 51. ISSN 1538-4357.

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We present measurements of [Fe/H] and [α/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy (R∼3000 and 6000, respectively). We observed four fields in M31's stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [α/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31's inner halo, finding ⟨[Fe/H]⟩ = −1.08 ± 0.04 and ⟨[α/Fe]⟩ = 0.40 ± 0.03. Between 8 and 34 kpc, the inner halo has a steep [Fe/H] gradient (−0.025 ± 0.002 dex kpc⁻¹) and negligible [α/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halo that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31's halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Escala, Ivanna0000-0002-9933-9551
Kirby, Evan N.0000-0001-6196-5162
Gilbert, Karoline M.0000-0003-0394-8377
Wojno, Jennifer0000-0002-3233-3032
Cunningham, Emily C.0000-0002-6993-0826
Guhathakurta, Puragra0000-0001-8867-4234
Additional Information:© 2020 The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2020 May 2; revised 2020 August 28; accepted 2020 August 31; published 2020 October 12. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank the referee for their comments, which improved this paper. The authors would additionally like to thank Miles Cranmer and Erik Tollerud for insightful discussions regarding statistics and David Nidever for providing a catalog of APOGEE data for the Magellanic Clouds. We also thank Stephen Gwyn for reducing the photometry for slitmasks f109_1 and f123_1, and Jason Kalirai for the reduction of f130_1. I.E. acknowledges support from a National Science Foundation (NSF) Graduate Research Fellowship under grant No. DGE-1745301, in addition to a Carnegie-Princeton Fellowship through the Carnegie Observatories. This material is based upon work supported by the NSF under grants No. AST-1614081 (E.N.K., I.E.) AST-1614569 (K.M.G, J.W.), and AST-1412648 (P.G.). E.N.K. gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement, as well as funding from generous donors to the California Institute of Technology. E.C.C. is supported by a Flatiron Research Fellowship at the Flatiron Institute. The Flatiron Institute is supported by the Simons Foundation. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST- 0071048. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facility: Keck II/DEIMOS. - Software: astropy (Astropy Collaboration et al. 2013, 2018), emcee (Foreman-Mackey et al. 2013).
Group:Astronomy Department
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
NSF Graduate Research FellowshipDGE-1745301
Carnegie-Princeton FellowshipUNSPECIFIED
Carnegie ObservatoriesUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Flatiron InstituteUNSPECIFIED
Simons FoundationUNSPECIFIED
Subject Keywords:Andromeda galaxy ; Galaxy stellar halos ; Stellar abundances ; Galaxy abundances ; Galaxy formation
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Andromeda Galaxy (39); Galaxy stellar halos (598); Stellar abundances (1577); Galaxy abundances (574); Galaxy formation (595)
Record Number:CaltechAUTHORS:20200922-145833729
Persistent URL:
Official Citation:Ivanna Escala et al 2020 ApJ 902 51
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105478
Deposited By: Tony Diaz
Deposited On:22 Sep 2020 23:06
Last Modified:12 Oct 2020 19:22

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