Optical Star Formation Rate Indicators

Using integrated optical spectrophotometry for 412 star-forming galaxies at z~0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxies at z~0.1, we investigate the Hα λ6563, Hβ λ4861, [O II] λ3727, and [O III] λ5007 nebular emission lines and the U-band luminosity as quantitative star formation rate (SFR) indicators. We demonstrate that the extinction-corrected Hα λ6563 luminosity is a reliable SFR tracer even in highly obscured star-forming galaxies. We find that variations in dust reddening dominate the systematic uncertainty in SFRs derived from the observed Hβ, [O II], and U-band luminosities, producing a factor of ~1.7, ~2.5, and ~2.1 scatter in the mean transformations, respectively. We show that [O II] depends weakly on variations in oxygen abundance over a wide range in metallicity, 12+log(O/H)=8.15-8.7 (Z/Z_solar=0.28-1.0), and that in this metallicity interval galaxies occupy a narrow range in ionization parameter (-3.8<~logU<~-2.9). We show that the scatter in [O III] λ5007 as a SFR indicator is a factor of 3-4 due to its sensitivity to metal abundance and ionization. We develop empirical SFR calibrations for Hβ and [O II] parameterized in terms of the B-band luminosity, which remove the systematic effects of reddening and metallicity and reduce the SFR scatter to +/-40% and +/-90%, respectively, although individual galaxies may deviate substantially from the median relations. Finally, we compare the z~0 relations between blue luminosity and reddening, ionization, and [O II]/Hα ratio against measurements at z~1 and find broad agreement. We emphasize, however, that optical emission-line measurements including Hα for larger samples of intermediate- and high-redshift galaxies are needed to test the applicability of our locally derived SFR calibrations to distant galaxies.