Optical star formation rate indicators

Using integrated optical spectrophotometry for 412 star-forming galaxies at z similar to 0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxies at z similar to 0.1, we investigate the H alpha lambda 6563, H beta lambda 4861, [O II] lambda 3727, and [O III] lambda 5007 nebular emission lines and the U-band luminosity as quantitative star formation rate (SFR) indicators. We demonstrate that the extinction-corrected H alpha lambda 6563 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 H beta, [O II], and U-band luminosities, producing a factor of similar to 1.7, similar to 2.5, and similar to 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(circle dot) = 0.28-1.0), and that in this metallicity interval galaxies occupy a narrow range in ionization parameter (-3.8 less than or similar to log U less than or similar to -2.9). We show that the scatter in [O III] lambda 5007 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 H beta 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 similar to 0 relations between blue luminosity and reddening, ionization, and [O II]/H alpha ratio against measurements at z similar to 1 and find broad agreement. We emphasize, however, that optical emission-line measurements including H alpha for larger samples of intermediate- and high-redshift galaxies are needed to test the applicability of our locally derived SFR calibrations to distant galaxies.