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

Journal Article · Thu Jul 23 00:00:00 EDT 2015 · The Astrophysical Journal (Online)

DOI:https://doi.org/10.1088/0004-637X/808/1/95· OSTI ID:1212743

Simon, J. D. ^[1]

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1. Carnegie Observatories, Pasadena, CA (United States). et al.

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 0°2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), High Energy Physics (HEP)
• Contributing Organization: DES Collaboration
• Grant/Contract Number: AC02-07CH11359; AC02-76SF00515
• OSTI ID: 1212743
• Alternate ID(s): OSTI ID: 1335100
• Report Number(s): FERMILAB-PUB-15-145-AE; SLAC-PUB-16733; arXiv:1504.02889; arXiv eprint number arXiv:1504.02889
• Journal Information: The Astrophysical Journal (Online), Vol. 808, Issue 1; ISSN 1538-4357
• Publisher: Institute of Physics (IOP)
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
dark matter
galaxies: dwarf
galaxies: individual (Reticulum II)
galaxies: stellar content
Local Group
stars: abundances
astrophysics
ASTRO