RE-EXAMINING HIGH ABUNDANCE SLOAN DIGITAL SKY SURVEY MASS-METALLICITY OUTLIERS: HIGH N/O, EVOLVED WOLF-RAYET GALAXIES?

Abstract

We present new MMT spectroscopic observations of four dwarf galaxies\nrepresentative of a larger sample observed by the Sloan Digital Sky Survey\n(SDSS) and identified by Peeples et al. (2008) as low-mass, high oxygen\nabundance outliers from the mass-metallicity relation. Peeples et al. (2008)\nshowed that these four objects (with metallicity estimates of 8.5 =< 12 +\nlog(O/H) =< 8.8) have oxygen abundance offsets of 0.4-0.6 dex from the M_B\nluminosity-metallicity relation. Our new observations extend the wavelength\ncoverage to include the [OII] 3726,3729 doublet, which adds leverage in oxygen\nabundance estimates and allows measurements of N/O ratios. All four spectra are\nlow excitation, with relatively high N/O ratios (N/O >~ 0.10), each of which\ntend to bias estimates based on strong emission lines toward high oxygen\nabundances. These spectra all fall in a regime where the "standard" strong line\nmethods for metallicity determinations are not well calibrated either\nempirically or by photoionization modeling. By comparing our spectra directly\nto photoionization models, we estimate oxygen abundances in the range of 7.9 =<\n12 + log(O/H) =< 8.4, consistent with the scatter of the mass-metallicity\nrelation. We discuss the physical nature of these galaxies that leads to their\nunusual spectra (and previous classification as outliers), finding their low\nexcitation, elevated N/O, and strong Balmer absorption are consistent with the\nproperties expected from galaxies evolving past the "Wolf-Rayet galaxy" phase.\nWe compare our results to the "main" sample of Peeples et al. (2008) and\nconclude that they are outliers primarily due to enrichment of nitrogen\nrelative to oxygen, and not due to unusually high oxygen abundances for their\nmasses or luminosities.\n