AEGIS: Extinction and Star Formation Tracers from Line Emission

Strong nebular emission lines are a sensitive probe of star formation and extinction in galaxies, and the [O II] line detects star-forming populations out to z > 1. However, star formation rates from emission lines depend on calibration of extinction and the [O II]/Hα line ratio, and separating star formation from AGN emission. We use calibrated line luminosities from the DEEP2 survey and Palomar K magnitudes to show that the behavior of emission-line ratios depends on galaxy magnitude and color. For galaxies on the blue side of the color bimodality, the vast majority show emission signatures of star formation, and there are strong correlations of extinction and [O II]/Hα with rest-frame H magnitude. The conversion of [O II] to extinction-corrected Hα and thus to star formation rate has a significant slope with MH, 0.23 dex mag-1. Red galaxies with emission lines have a much higher scatter in their line ratios, and more than half show AGN signatures. We use 24 μm fluxes from Spitzer MIPS to demonstrate the differing populations probed by nebular emission and by mid-IR luminosity. Although extinction is correlated with luminosity, 98% of IR-luminous galaxies at z ~ 1 are still detected in the [O II] line. Mid-IR-detected galaxies are mostly bright and intermediate color, while fainter, bluer galaxies with high [O II] luminosity are rarely detected at 24 μm.