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