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 [ Fe / H ] < -3.0 of stars in a dwarf galaxy , with individual stellar metallicities as low as [ Fe / H ] = -3.3 .
Because 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 .