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

J. D. Simon, A. Drlica-Wagner, T. S. Li, B. Nord, M. Geha, K. Bechtol, E. Balbinot, E. Buckley-Geer, H. Lin, J. Marshall, B. Santiago, L. Strigari, M. Wang, R. H. Wechsler, B. Yanny, T. Abbott, A. H. Bauer, G. M. Bernstein, E. Bertin, D. BrooksD. L. Burke, D. Capozzi, A. Carnero Rosell, M. Carrasco Kind, C. B. D'Andrea, L. N. Da Costa, D. L. DePoy, S. Desai, H. T. Diehl, S. Dodelson, C. E. Cunha, J. Estrada, A. E. Evrard, A. Fausti Neto, E. Fernandez, D. A. Finley, B. Flaugher, J. Frieman, E. Gaztanaga, D. Gerdes, D. Gruen, R. A. Gruendl, K. Honscheid, D. James, S. Kent, K. Kuehn, N. Kuropatkin, O. Lahav, M. A. G. Maia, M. March, Dark Energy Survey Collaboration

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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 ± 0.5 km s-1 and a velocity dispersion of 3.3 ± 0.7 km s-1. The mass-to-light ratio of Ret II within its half-light radius is 470 ± 210 M/L, 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 km 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 [Fe/H] < -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 [Fe/H] = -2.65 ± 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 log10(J) = 18.8 ± 0.6 GeV2 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.

Original languageEnglish
Article number95
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 23 Jul 2015


  • dark matter
  • galaxies: dwarf
  • galaxies: individual (Reticulum II)
  • galaxies: stellar content
  • Local Group
  • stars: bundances
  • RCUK
  • STFC


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