Choirs HI Galaxy Groups: The metallicity of dwarf galaxies
  We present a recalibration of the luminosity-metallicity relation for
gas-rich, star-forming dwarfs to magnitudes as faint as M$_R$ ~ -13. We use the
Dopita et al. (2013) metallicity calibrations to calibrate the relation for all
of the data in this analysis. In metallicity-luminosity space we find two
sub-populations within a sample of high-confidence SDSS DR8 star-forming
galaxies; 52% are metal-rich giants and 48% are metal-medium galaxies.
Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the
literature are typically of metallicity 12 + log(O/H) = 8.70 $\pm$ 0.05, while
SDSS dwarfs fainter than M$_R$ = -16 have a mean metallicity of 12 + log(O/H) =
8.28 $\pm$ 0.10, regardless of their luminosity, indicating that there is an
approximate floor to the metallicity of low luminosity galaxies. Our
hydrodynamical simulations predict that TDGs should have metallicities elevated
above the normal luminosity-metallicity relation. Metallicity can therefore be
a useful diagnostic for identifying TDG candidate populations in the absence of
tidal tails. At magnitudes brighter than M$_R$ ~ -16 our sample of 53
star-forming galaxies in 9 HI gas-rich groups is consistent with the normal
relation defined by the SDSS sample. At fainter magnitudes there is an increase
in dispersion in metallicity of our sample, suggestive of a wide range of HI
content and environment. In our sample we identify three (16% of dwarfs) strong
TDG candidates (12 + log(O/H) > 8.6), and four (21%) very metal poor dwarfs (12
+ log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star
formation.
