The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf
  Galaxies
  We present spectroscopic metallicities of individual stars in seven gas-rich
dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same
mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the
Milky Way and M31: Z_* ~ M_*^(0.30 +/- 0.02). The uniformity of the relation is
in contradiction to previous estimates of metallicity based on photometry. This
relationship is roughly continuous with the stellar mass-stellar metallicity
relation for galaxies as massive as M_* = 10^12 M_sun. Although the average
metallicities of dwarf galaxies depend only on stellar mass, the shapes of
their metallicity distributions depend on galaxy type. The metallicity
distributions of dIrrs resemble simple, leaky box chemical evolution models,
whereas dSphs require an additional parameter, such as gas accretion, to
explain the shapes of their metallicity distributions. Furthermore, the
metallicity distributions of the more luminous dSphs have sharp, metal-rich
cut-offs that are consistent with the sudden truncation of star formation due
to ram pressure stripping.
