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 H ii regions culled from the literature. We consider both theoretical and empirical strong-line abundance calibrations based on the R23 ≡ ([O ii] + [O iii])/Hβ 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 H ii-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 emissionline 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 [N ii]/Hα and [S ii]/Hα ratios are enhanced with respect to line-ratios typical of H ii 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. Subject headings: galaxies: abundances — galaxies: evolution — galaxies: spiral