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 .
( 53 ) as low-mass , high oxygen abundance outliers from the mass-metallicity relation . ( 53 ) showed that these four objects ( with metallicity estimates of 8.5 \leq 12 + \log { ( \mbox { O / H } ) } \leq 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 [ O ii ] \lambda \lambda 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 ( \mbox { N / O } \gtrsim 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 \leq 12 + \log { \mbox { ( O / H ) } } \leq 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 ( 53 ) 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 .