We study whether integrated optical spectroscopy of a disk galaxy can be usedto infer the mean, or characteristic gas-phase oxygen abundance in the presenceof systematic effects such as spatial abundance variations, contributions tothe integrated emission-line spectrum from diffuse-ionized gas, and dustattenuation. Our sample consists of 14 nearby disk galaxies with integratedspectrophotometry, and observations of more than 250 individual HII regionsculled from the literature. We consider both theoretical and empiricalstrong-line abundance calibrations based on the R23=([OII]+[OIII])/H-betaparameter. We find that the integrated oxygen abundance correlates well withthe gas-phase abundance measured at a fixed galactocentric radius, asdetermined by the HII-region abundance gradient. The typical scatter in thecorrelation is +/-0.1 dex, independent of the abundance calibration, or whetherthe observed integrated emission-line fluxes, the reddening-corrected fluxes,or the emission-line equivalent widths are used. Integrated abundances based onthe observed fluxes or equivalent widths, however, are susceptible toadditional systematic effects of order 0.05-0.1 dex, at least for the range ofreddenings and stellar populations spanned by our sample. Unlike the integratedR23 parameter, we find that the integrated [NII]/H-alpha and [SII]/H-alpharatios are enhanced with respect to line-ratios typical of HII regions,consistent with a modest contribution from diffuse-ionized gas emission. Weconclude that the R23 parameter can be used to reliably measure the gas-phaseabundances of distant star-forming galaxies.
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