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 dex (Z/Z⊙ = 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] 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 intermediateand high-redshift galaxies are needed to test the applicability of our locally derived SFR calibrations to distant galaxies. Subject headings: galaxies: abundances — galaxies: evolution — galaxies: formation — galaxies: ISM Steward Observatory, University of Arizona, 933 N Cherry Ave., Tucson, AZ 85721, USA Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom