Stellar Kinematics and Metallicities in the Ultra-Faint Dwarf Galaxy Reticulum II
Creators: name: Simon, J. D.
name: Drlica-Wagner, A.
name: Li, T. S.
name: Nord, B.
name: Geha, M.
name: Bechtol, K.
name: Balbinot, E.
name: Buckley-Geer, E.
name: Lin, H.
name: Marshall, J.
name: Santiago, B.
name: Strigari, L.
name: Wang, M.
name: Wechsler, R. H.
name: Yanny, B.
name: Abbott, T.
name: Bauer, A. H.
name: Bernstein, G. M.
name: Bertin, E.
name: Brooks, D.
name: Burke, D. L.
name: Capozzi, D.
name: Rosell, A. Carnero
name: Kind, M. Carrasco
name: D'Andrea, C. B.
name: da Costa, L. N.
name: DePoy, D. L.
name: Desai, S.
name: Diehl, H. T.
name: Dodelson, S.
name: Cunha, C. E
name: Estrada, J.
name: Evrard, A. E.
name: Neto, A. Fausti
name: Fernandez, E.
name: Finley, D. A.
name: Flaugher, B.
name: Frieman, J.
name: Gaztanaga, E.
name: Gerdes, D.
name: Gruen, D.
name: Gruendl, R. A.
name: Honscheid, K.
name: James, D.
name: Kent, S.
name: Kuehn, K.
name: Kuropatkin, N.
name: Lahav, O.
name: Maia, M. A. G.
name: March, M.
name: Martini, P.
name: Miller, C. J.
name: Miquel, R.
name: Ogando, R.
name: Romer, A. K.
name: Roodman, A.
name: Rykoff, E. S.
name: Sako, M.
name: Sanchez, E.
name: Schubnell, M.
name: Sevilla, I.
name: Smith, R. C.
name: Soares-Santos, M.
name: Sobreira, F.
name: Suchyta, E.
name: Swanson, M. E. C.
name: Tarle, G.
name: Thaler, J.
name: Tucker, D.
name: Vikram, V.
name: Walker, A. R.
name: Wester, W.
Publisher: name: arXiv
Year: 2015
Subjects: subject: Astrophysics of Galaxies (astro-ph.GA)
subject: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present Magellan/M2FS, VLT/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 and a velocity dispersion of 3.3 +/- 0.7 km/s. The mass-to-light ratio of Ret II within its half-light radius is 470 +/- 210 Msun/Lsun, 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, 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] &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 [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 log J = 18.8 +/- 0.6 Gev^2/cm^5 within 0.2 degrees, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.
description: 14 pages, 6 figures (4 in color), 2 tables. Accepted for publication in the Astrophysical Journal. Only minor changes from v1