STELLAR KINEMATICS AND METALLICITIES IN THE ULTRA-FAINT DWARF GALAXY RETICULUM II Academic Article uri icon

abstract

  • 2015. The American Astronomical Society. All rights reserved. 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.

published proceedings

  • ASTROPHYSICAL JOURNAL

altmetric score

  • 29.8

author list (cited authors)

  • Simon, J. D., Drlica-Wagner, A., Li, T. S., Nord, B., Geha, M., Bechtol, K., ... Wester, W.

citation count

  • 128

complete list of authors

  • Simon, JD||Drlica-Wagner, A||Li, TS||Nord, B||Geha, M||Bechtol, K||Balbinot, E||Buckley-Geer, E||Lin, H||Marshall, J||Santiago, B||Strigari, L||Wang, M||Wechsler, RH||Yanny, B||Abbott, T||Bauer, AH||Bernstein, GM||Bertin, E||Brooks, D||Burke, DL||Capozzi, D||Carnero Rosell, A||Kind, M Carrasco||D'Andrea, CB||da Costa, LN||DePoy, DL||Desai, S||Diehl, HT||Dodelson, S||Cunha, CE||Estrada, J||Evrard, AE||Fausti Neto, A||Fernandez, E||Finley, DA||Flaugher, B||Frieman, J||Gaztanaga, E||Gerdes, D||Gruen, D||Gruendl, RA||Honscheid, K||James, D||Kent, S||Kuehn, K||Kuropatkin, N||Lahav, O||Maia, MAG||March, M||Martini, P||Miller, CJ||Miquel, R||Ogando, R||Romer, AK||Roodman, A||Rykoff, ES||Sako, M||Sanchez, E||Schubnell, M||Sevilla, I||Smith, RC||Soares-Santos, M||Sobreira, F||Suchyta, E||Swanson, MEC||Tarle, G||Thaler, J||Tucker, D||Vikram, V||Walker, AR||Wester, W

publication date

  • July 2015