We investigate the instantaneous star formation rates ( SFR ) and extinction properties for a large ( N = 274 ) , near-infrared ( NIR : 2.2µm ) + mid-infrared ( MIR : 24µm ) selected sample of normal to ultra-luminous infrared galaxies ( ULIRGs ) [ 10 ^ { 9 } < L _ { IR } / L _ { \odot } < 10 ^ { 12.5 } ] with \langle z \rangle \sim 0.8 in the Spitzer Extragalactic First Look Survey ( FLS ) .
We combine Spitzer MIPS 24µm observations with high-resolution , optical Keck Deimos spectroscopy to derive optical emission-line ( H \alpha , H \beta , [ OII ] ) 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 < A _ { v } < 4.0 mag ) .
This is after removing spectroscopic and IRAC color-selected AGN candidates that account for \approx 12 \% of the sample .
Objects with SFRs of a few solar masses per year have A _ { v } values consistent with those of normal spirals ( A _ { v } \approx 1.0 mag ) .
By contrast , LIRGs at z \gtrsim 1 , which make up a large fraction of our sample , have SFR \approx 100 M _ { \odot } yr ^ { -1 } and a mean A _ { v } \approx 2.5 mag .
This translates to a 97 % mean attenuation of the [ OII ] \lambda \lambda 3727 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 A _ { v } ^ { IR } function [ A _ { v } ^ { IR } = 0.75 *log ( L _ { IR } / L _ { \odot } ) -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 ( Semi-Interquartile Range ) .
Investigation of the A _ { v } dependence on redshift reveals that for a fixed L _ { IR } there is no significant A _ { v } evolution .
Comparisons to previous studies reveal that 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 .