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Multiple Chemodynamic Stellar Populations of the Ursa Minor Dwarf Spheroidal Galaxy

Pace, Andrew B. and Kaplinghat, Manoj and Kirby, Evan and Simon, Joshua D. and Tollerud, Erik and Muñoz, Ricardo R. and Côté, Patrick and Djorgovski, S. G. and Geha, Marla (2020) Multiple Chemodynamic Stellar Populations of the Ursa Minor Dwarf Spheroidal Galaxy. Monthly Notices of the Royal Astronomical Society, 495 (3). pp. 3022-3040. ISSN 0035-8711.

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We present a Bayesian method to identify multiple (chemodynamic) stellar populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity, and positional stellar data without the assumption of spherical symmetry. We apply this method to a new Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS) spectroscopic survey of the Ursa Minor (UMi) dSph. We identify 892 likely members, making this the largest UMi sample with line-of-sight velocity and metallicity measurements. Our Bayesian method detects two distinct chemodynamic populations with high significance (in logarithmic Bayes factor, ln B ∼ 33). The metal-rich ([Fe/H] = −2.05 ± 0.03) population is kinematically colder (radial velocity dispersion of σ_v = 4.9^(+0.8)_(−1.0)kms⁻¹⁠) and more centrally concentrated than the metal-poor (⁠[Fe/H] = −2.29^(+0.05)_(−0.06)⁠) and kinematically hotter population (⁠σ_v = 11.5^(+0.9)_(−0.8)kms⁻¹⁠). Furthermore, we apply the same analysis to an independent Multiple Mirror Telescope (MMT)/Hectochelle data set and confirm the existence of two chemodynamic populations in UMi. In both data sets, the metal-rich population is significantly flattened (ϵ = 0.75 ± 0.03) and the metal-poor population is closer to spherical (⁠ϵ = 0.33^(+0.12)_(−0.09)⁠). Despite the presence of two populations, we are able to robustly estimate the slope of the dynamical mass profile. We found hints for prolate rotation of order ∼2kms⁻¹ in the MMT data set, but further observations are required to verify this. The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators, so we computed new astrophysical dark matter annihilation (J) and decay profiles based on the rounder, hotter metal-poor population and inferred log₁₀(J(0.∘5)/GeV²cm⁻⁵) ≈ 19.1 for the Keck data set. Our results paint a more complex picture of the evolution of UMi than previously discussed.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Pace, Andrew B.0000-0002-6021-8760
Kirby, Evan0000-0001-6196-5162
Tollerud, Erik0000-0002-9599-310X
Côté, Patrick0000-0003-1184-8114
Djorgovski, S. G.0000-0002-0603-3087
Geha, Marla0000-0002-7007-9725
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 May 18. Received 2020 May 11; in original form 2020 February 21. Published: 22 May 2020. We thank Matt Walker for sharing an earlier catalogue of the UMi MMT/Hectochelle data. We thank James Bullock, Mike Cooper, Sergey Koposov, Jen Marshall, Louie Strigari, and Matt Walker for helpful comments and discussion. We also thank the referee for their helpful comments. ABP was supported by a GAANN fellowship at UCI. ABP acknowledges generous support from the George P. and Cynthia Woods Institute for Fundamental Physics and Astronomy at Texas A&M University. ABP was supported by NSF grant AST-1813881. EK gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement and funding from generous donors to the California Institute of Technology. RRM acknowledges partial support from project BASAL AFB-170002 and FONDECYT project N°1170364. SGD was supported in part by the NSF grants AST-1413600, AST-1518308, and AST-1749235. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science. The SDSS-III web site is SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of NASA’s Astrophysics Data System Bibliographic Services. Databases and software: Besançon model15 (Robin et al. 2003). PYTHON packages: ASTROPY16 (Astropy Collaboration 2013), NUMPY (Walt, Colbert & Varoquaux 2011), IPYTHON (Pérez & Granger 2007), SCIPY (Jones et al. 2001), MATPLOTLIB (Hunter 2007), CORNER.PY (Foreman-Mackey 2016), and EMCEE (Foreman-Mackey et al. 2013). The data presented in this work 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.
Group:Astronomy Department
Funding AgencyGrant Number
University of California, IrvineUNSPECIFIED
Texas A&M UniversityUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1170364
Alfred P. Sloan FoundationUNSPECIFIED
Participating InstitutionsUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Gaia Multilateral AgreementUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:galaxies: evolution – galaxies: kinematics and dynamics – Local Group – cosmology: dark matter – galaxies: individual: Ursa Minor dSph
Issue or Number:3
Record Number:CaltechAUTHORS:20200414-071817157
Persistent URL:
Official Citation:Andrew B Pace, Manoj Kaplinghat, Evan Kirby, Joshua D Simon, Erik Tollerud, Ricardo R Muñoz, Patrick Côté, S G Djorgovski, Marla Geha, Multiple chemodynamic stellar populations of the Ursa Minor dwarf spheroidal galaxy, Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 3, July 2020, Pages 3022–3040,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102517
Deposited By: Tony Diaz
Deposited On:14 Apr 2020 16:27
Last Modified:17 Jul 2020 16:40

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