The Metallicities of Low Stellar Mass Galaxies and the Scatter in the
  Mass-Metallicity Relation
  In this investigation we quantify the metallicities of low mass galaxies by
constructing the most comprehensive census to date. We use galaxies from the
SDSS and DEEP2 survey and estimate metallicities from their optical emission
lines. We also use two smaller samples from the literature which have
metallicities determined by the direct method using the temperature sensitive
[OIII]4363 line. We examine the scatter in the local mass-metallicity (MZ)
relation determined from ~20,000 star-forming galaxies in the SDSS and show
that it is larger at lower stellar masses, consistent with the theoretical
scatter in the MZ relation determined from hydrodynamical simulations. We
determine a lower limit for the scatter in metallicities of galaxies down to
stellar masses of ~10^7 M_solar that is only slightly smaller than the expected
scatter inferred from the SDSS MZ relation and significantly larger than what
is previously established in the literature. The average metallicity of
star-forming galaxies increases with stellar mass. By examining the scatter in
the SDSS MZ relation, we show that this is mostly due to the lowest metallicity
galaxies. The population of low mass, metal-rich galaxies have properties which
are consistent with previously identified galaxies that may be transitional
objects between gas-rich dwarf irregulars and gas-poor dwarf spheroidals and
ellipticals.
