True chemical abundances of galaxies in the nearby universe: a comparison of abundance methods, interstellar processes, and galaxy types

Abstract

Peeples et al. (2008) identified low-mass, high oxygen abundance outliers from the mass-metallicity (M-Z) relationship. We present new MMT spectroscopy of four of these dwarf galaxy outliers. We re-examined these anomalous spectra and compared to the parameter space for which standard strong-line methods are calibrated. 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. To address the issue of securing the low-luminosity end of the M-Z relationship, we present MMT spectroscopic observations of HII regions in 42 low-luminosity galaxies in the Spitzer LVL survey. Direct oxygen abundances were determined based on the temperature sensitive [O III] λ4363 line, measured at a strength of 4σ or greater, for 31 of the 42 galaxies in our sample. Combining our results with previous direct abundance studies, we present a further refined sample, requiring reliable distance determinations. We characterize the direct L-Z and M-Z relationships at low-luminosity using the resulting 38 object sample. We show that the luminosity of a low-luminosity galaxy is often a better indicator of metallicity than strong-line methods. Additionally, our results provide the first direct estimates of oxygen abundance for 19 local volume dwarf galaxies. Properties of the ISM of spiral galaxies are known to show radial variations. Motivated by the need to place gradients on the same scale for comparisons amongst galaxies, we present direct oxygen abundance gradients of the nearby spiral galaxies NGC 628 and NGC 2403. A bi-modal N/O gradient pattern is measured for NGC 628. Notably, the N/O ratio plateaus beyond R₂₅, demonstrating that primary nitrogen production is the dominant mechanism in the outer disk. The outer disk beyond R₂₅ was not sampled in NGC 2403. As expected for α-process elements, we show that S/O, Ne/O, and Ar/O appear to be constant over a range in oxygen abundance in both NGC 628 and NGC 2403 such that the α-elements and O are produced in lock-step.