Optical Star-Formation Rate Indicators.pdf

a r X i v : a s t r o - p h / 0 5 1 1 7 3 0 v 2 8 J a n 2 0 0 6 ApJ, accepted Preprint typeset using LATEX style emulateapj v. 6/22/04 OPTICAL STAR-FORMATION RATE INDICATORS John Moustakas1, Robert C. Kennicutt, Jr.1,2, Christy A. Tremonti1 ApJ, accepted ABSTRACT 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 intermediate- and high-re