Re-examining High Abundance SDSS Mass-Metallicity Outliers: High N/O,
  Evolved Wolf-Rayet Galaxies?
  We present new MMT spectroscopic observations of four dwarf galaxies
representative of a larger sample observed by the Sloan Digital Sky Survey
(SDSS) and identified by Peeples et al. (2008) as low-mass, high oxygen
abundance outliers from the mass-metallicity relation. Peeples et al. (2008)
showed that these four objects (with metallicity estimates of 8.5 =< 12 +
log(O/H) =< 8.8) have oxygen abundance offsets of 0.4-0.6 dex from the M_B
luminosity-metallicity relation. Our new observations extend the wavelength
coverage to include the [OII] 3726,3729 doublet, which adds leverage in oxygen
abundance estimates and allows measurements of N/O ratios. All four spectra are
low excitation, with relatively high N/O ratios (N/O >~ 0.10), each of which
tend to bias estimates based on strong emission lines toward high oxygen
abundances. These spectra all fall in a regime where the "standard" strong line
methods for metallicity determinations are not well calibrated either
empirically or by photoionization modeling. By comparing our spectra directly
to photoionization models, we estimate oxygen abundances in the range of 7.9 =<
12 + log(O/H) =< 8.4, consistent with the scatter of the mass-metallicity
relation. We discuss the physical nature of these galaxies that leads to their
unusual spectra (and previous classification as outliers), finding their low
excitation, elevated N/O, and strong Balmer absorption are consistent with the
properties expected from galaxies evolving past the "Wolf-Rayet galaxy" phase.
We compare our results to the "main" sample of Peeples et al. (2008) and
conclude that they are outliers primarily due to enrichment of nitrogen
relative to oxygen, and not due to unusually high oxygen abundances for their
masses or luminosities.
