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 H-alpha, H-beta, O II 3727, and O III 5007 nebular emission lines and the U-band luminosity as quantitative star-formation rate (SFR) indicators. We demonstrate that the extinction-corrected H-alpha 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 ~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 dex (Z/Zₛun=0. 28-1. 0), and that in this metallicity interval galaxies occupy a narrow range in ionization parameter (-3. 8<log U<-2. 9 dex). We show that the scatter in O III 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~0 relations between blue luminosity and reddening, ionization, and O II/H-alpha ratio against measurements at z~1 and find broad agreement. (Abridged. )