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Star Formation Rates and Extinction Properties of IR-Luminous Galaxies in the Spitzer First Look Survey

We investigate the instantaneous star formation rates (SFR) and extinction properties for a large (N=274), near-infrared (2.2 micron) + mid-infrared (24 micron) selected sample of normal to ultra-luminous infrared galaxies (ULIRGs) [10^9<L_{IR}/L_{sun}<10^12.5] with <z>~0.8 in the Spitzer Extragalactic First Look Survey. We combine Spitzer MIPS 24-micron observations with high-resolution, optical Keck Deimos spectroscopy to derive optical emission-line and infrared star formation rates (SFR_{opt} & SFR_{IR}, respectively). Direct comparison of these SFR diagnostics reveals that our sample exhibits a wide range of extinction (1.0<Av<4.0 mag). This is after removing spectroscopic and IRAC color-selected AGN candidates that account for 12% of the sample. Objects with SFRs of a few solar masses per year have Av values consistent with those of normal spirals (Av~1.0 mag). By contrast, LIRGs at z>1, which make up a large fraction of our sample, have SFR~100 M_{sun}/yr and <Av>~2.5 mag. This translates to a 97% mean attenuation of the [OII] forbidden line doublet, with the most extreme sources having as much as 99.7% of their [OII] line flux extinguished by dust. Based on a SFR_{IR}/SFR_{opt} diagnostic, we derive an IR-luminosity-dependent Av^{IR} function [Av^{IR}=0.75*log(L_{IR}/L_{sun})-6.35 mag] that we use to extinction correct our emission line luminosities. Application of this correction results in a correlation between SFR_{IR} and SFR_{opt} that has a dispersion of 0.2 dex. Investigation of the Av dependence on redshift reveals that for a fixed L_{IR}, there is no significant Av evolution. The mean attenuation of our sample is intermediate between that of local optical/UV- and radio-selected samples and has a marginally stronger L_{IR} dependence.