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Title: Stellar kinematics and metallicities in the ultra-faint dwarf galaxy Reticulum II

With this study, we present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of $$62.8\pm 0.5\;\mathrm{km}\;{{\rm{s}}}^{-1}$$ and a velocity dispersion of $$3.3\pm 0.7\;\mathrm{km}\;{{\rm{s}}}^{-1}$$. The mass-to-light ratio of Ret II within its half-light radius is $$470\pm 210\ {M}_{\odot }/{L}_{\odot }$$, demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 $$\mathrm{km}\ {{\rm{s}}}^{-1}$$, respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 ± 0.09 dex, and we identify several extremely metal-poor stars with $${\rm{[Fe/H]}}\lt -3$$. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of $${\rm{[Fe/H]}}=-2.65\pm 0.07$$, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is $${\mathrm{log}}_{10}(J)=18.8\pm 0.6\;\;\mathrm{GeV}{\;}^{2}\;{\mathrm{cm}}^{-5}\;$$ within 0fdg2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.
  1. Carnegie Observatories, Pasadena, CA (United States). et al.
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 1538-4357; arXiv eprint number arXiv:1504.02889
Grant/Contract Number:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 808; Journal Issue: 1; Journal ID: ISSN 1538-4357
Institute of Physics (IOP)
Research Org:
Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Orgs:
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS dark matter; galaxies: dwarf; galaxies: individual (Reticulum II); galaxies: stellar content; Local Group; stars: abundances