The DEEP Groth Strip Survey XII: The Metallicity of Field Galaxies at z=0.26-0.82

Using spectroscopic data from the DEEP Groth Strip Survey (DGSS), we analyze the gas-phase oxygen abundances for 66 field galaxies in the redshift range z=0.26-0.82. We develop a prescription for estimating the nebular oxygen abundance of distant galaxies using the equivalent width of strong [O II], [O III], and Balmer emission lines. Oxygen abundances for the DGSS sample, relative to hydrogen, range between 8.4<12+log(O/H)<9.0 with typical uncertainties of 0.10 dex. The 66 DGSS galaxies collectively exhibit a correlation between B-band luminosity and metallicity. Subsets of DGSS galaxies binned by redshift also exhibit correlations but with different zero points. Galaxies in the highest redshift bin (z=0.6-0.82) are brighter by ~1 mag compared to the lowest redshift bin (z=0.6-0.82) and brighter by ~1-2 mag compared to local (z<0.1) field galaxies. This offset from the local L-Z relation for field galaxies is greatest for objects at the low-luminosity (M_{B ~-19}) end of the sample, and vanishingly small for objects at the high-luminosity end of the sample (M_{B ~-22}). This result implies that emission-line field galaxies have undergone moderate amounts of evolution in the past ~7 Gyr since z ~0.8, and that the evolution is most significant for galaxies of lower luminosity. Either the least-luminous DGSS field galaxies have faded by 1--2 mag due to decreasing levels of star formation, or they have experienced an increase in the mean metallicity of the interstellar medium by factors of 1.3--2 (0.1-0.3 dex). Some combination of the two processes is likely.