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

Abstract

We present new metallicity measurements for 298 individual red giant branch\nstars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the\nMilky Way (MW) system. Our technique is based on medium resolution Keck/DEIMOS\nspectroscopy coupled with spectral synthesis. We present the first\nspectroscopic metallicities at [Fe/H] < -3.0 of stars in a dwarf galaxy, with\nindividual stellar metallicities as low as [Fe/H] = -3.3. Because our [Fe/H]\nmeasurements are not tied to empirical metallicity calibrators and are\nsensitive to arbitrarily low metallicities, we are able to probe this extremely\nmetal-poor regime accurately. The metallicity distribution of stars in these\ndSphs is similar to the MW halo at the metal-poor end. We also demonstrate that\nthe luminosity-metallicity relation previously seen in more luminous dSph\ngalaxies (M_V = -13.4 to -8.8) extends smoothly down to an absolute magnitude\nof M_V = -3.7. The discovery of extremely metal-poor stars in dSphs lends\nsupport to the LCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to\nform the stellar halo of the MW.\n