Measuring global galaxy metallicities using emission-line equivalent widths

We develop a prescription for estimating the interstellar medium oxygen abundances of distant star-forming galaxies using the ratio EW R-23 formed from the equivalent widths of the [O II] lambda3727, [O III] lambdalambda4959, 5007, and Hbeta nebular emission lines. This EW R-23 approach is essentially identical to the widely used R-23 method of Pagel and coworkers. Using data from three spectroscopic surveys of nearby galaxies, we conclude that the emission-line equivalent-width ratios are a good substitute for emission-line flux ratios in galaxies with active star formation. The rms dispersion between EW R-23 and the reddening-corrected R-23 values is sigma(log R-23) less than or equal to 0.08 dex. This dispersion is comparable to the emission-line measurement uncertainties for distant galaxies in many spectroscopic galaxy surveys, and is somewhat smaller than the uncertainties of sigma(O/H) similar to 0.15 dex inherent in strong-line metallicity calibrations. Because equivalent-width ratios are partially insensitive to interstellar reddening effects, emission-line equivalent-width ratios should be superior to flux ratios when reddening corrections are not available. The EW R-23 method presented here is likely to be most useful for statistically estimating the mean metallicities for large samples of galaxies to trace their chemical properties as a function of redshift or environment. The approach developed here is used in a companion paper by Kobulnicky and coworkers to measure the metallicities of star-forming galaxies at z = 0.2-0.8 in the Deep Extragalactic Evolutionary Probe spectroscopic survey of the Groth strip.