Integrated Nebular Abundances of Disk Galaxies

We study whether integrated optical spectroscopy of a disk galaxy can be used to infer the mean, or characteristic gas-phase oxygen abundance in the presence of systematic effects such as spatial abundance variations, contributions to the integrated emission-line spectrum from diffuse-ionized gas, and dust attenuation. Our sample consists of 14 nearby disk galaxies with integrated spectrophotometry, and observations of more than 250 individual HII regions culled from the literature. We consider both theoretical and empirical strong-line abundance calibrations based on the R23=([OII]+[OIII])/H-beta parameter. We find that the integrated oxygen abundance correlates well with the gas-phase abundance measured at a fixed galactocentric radius, as determined by the HII-region abundance gradient. The typical scatter in the correlation is +/-0.1 dex, independent of the abundance calibration, or whether the observed integrated emission-line fluxes, the reddening-corrected fluxes, or the emission-line equivalent widths are used. Integrated abundances based on the observed fluxes or equivalent widths, however, are susceptible to additional systematic effects of order 0.05-0.1 dex, at least for the range of reddenings and stellar populations spanned by our sample. Unlike the integrated R23 parameter, we find that the integrated [NII]/H-alpha and [SII]/H-alpha ratios are enhanced with respect to line-ratios typical of HII regions, consistent with a modest contribution from diffuse-ionized gas emission. We conclude that the R23 parameter can be used to reliably measure the gas-phase abundances of distant star-forming galaxies.