Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies

Kirby, Evan N.
Simon, Joshua D.
Geha, Marla
Guhathakurta, Puragra
Frebel, Anna
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We present new metallicity measurements for 298 individual red giant branch stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the Milky Way ( MW) system. Our technique is based on medium-resolution Keck DEIMOS spectroscopy coupled with spectral synthesis. We present the first spectroscopic metallicities at of stars in a dwarf galaxy, with individual stellar metallicities as low as. Because [Fe/H] < -3.0 [Fe/H] = -3.3 our [Fe/H] measurements are not tied to empirical metallicity calibrators and are sensitive to arbitrarily low metallicities, we are able to probe this extremely metal-poor regime accurately. The metallicity distribution of stars in these dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that the luminosity-metallicity relation previously seen in more luminous dSph galaxies (M(v) = -13.4 to -8.8) extends smoothly down to an absolute magnitude of M(v) = -3.7. The discovery of extremely metal-poor stars in dSphs lends support to the Lambda CDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW.

Kirby, Evan N., Joshua D. Simon, Marla Geha, Puragra Guhathakurta, and Anna Frebel. "Uncovering Extremely Metal-Poor Stars in the Milky Way’s Ultrafaint Dwarf Spheroidal Satellite GalaxiesData herein were obtained at the WM Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the WM Keck Foundation." The Astrophysical Journal Letters, Vol. 685, No. 1 (Sep., 2008): L43.