Star formation rates and extinction properties of IR-luminous galaxies in the Spitzer First Look Survey

We investigate the instantaneous star formation rates (SFRs) and extinction properties for a large (N = 274), near-infrared (NIR: 2.2 mu m) + mid-infrared (MIR: 24 mu m)-selected sample of normal to ultraluminous infrared galaxies (ULIRGs; 10(9) < L-IR/L-circle dot < 10(12.5)) with < z > similar to 0.8 in the Spitzer Extragalactic First Look Survey (FLS). We combine 24 mu m observations with high-resolution Keck DEIMOS spectroscopy to derive optical emission-line (H alpha, H beta, and [O II]) and infrared star formation rates (SFRopt and SFRIR, respectively). Comparison of SFR diagnostics reveals a wide extinction range (1.0 < A(V) < 4.0 mag) for this sample, even after removing spectroscopic and IRAC color-selected AGN candidates (approximate to 12% of the sample). Objects with SFRs of a few M-circle dot yr(-1) have extinction values consistent with normal spirals (A(V) approximate to 1.0 mag). By contrast, LIRGs at z greater than or similar to 1, which comprise a fraction of our sample, have SFR approximate to 100 M-circle dot yr(-1) and a mean A(V) approximate to 2.5 mag. This translates to a 97% mean [O II] lambda lambda 3727 attentuation and in extreme cases is as high as 99.7%. We derive an IR-luminosity-dependent A(V)(IR) function [A(V)(IR) = 0.75 log (L-IR/L-circle dot) -6.35 mag] that we use to extinction correct our line luminosities. The resulting correlation between SFRIR and SFRopt has a dispersion of similar to 0.2 dex (semi-interquartile range). Comparison of the A(V) dependence on redshift and L-IR reveals that for a fixed L-IR, there is no significant A(V) evolution. Comparison to previous studies reveals a mean attenuation that is intermediate between that of local optical/UV- and radio-selected samples with a marginally stronger L-IR dependence.