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 km) + mid-infrared (MIR: 24 km)-selected sample of normal to ultraluminous infrared galaxies (ULIRGs; 10 super(9) < L sub(IR)/L sub( )< 10 super(12.5)) with < z > 6 0.8 in the Spitzer Extragalactic First Look Survey (FLS). We combine 24 km observations with high-resolution Keck DEIMOS spectroscopy to derive optical emission-line (Ha, Hb, and [O II]) and infrared star formation rates (SFR sub(opt) and SFR sub(IR), respectively). Comparison of SFR diagnostics reveals a wide extinction range (1.0 < A sub(V) < 4.0 mag) for this sample, even after removing spectroscopic and IRAC color-selected AGN candidates (- 12% of the sample). Objects with SFRs of a few M sub( )yr super(-1) have extinction values consistent with normal spirals (A sub(V) - 1.O mag). By contrast, LIRGs at z 1, which comprise a fraction of our sample, have SFR - 100 M sub( )yr super(-1) and a mean A sub(V) - 2.5 mag. This translates to a 97% mean [O II] ll3727 attentuation and in extreme cases is as high as 99.7%. We derive an IR-luminosity-dependent A super(I) sub(v) super(R) function [A super(I) sub(v) super(R) = 0.75 log(L sub(IR)/L sub( )) - 6.35 mag] that we use to extinction correct our line luminosities. The resulting correlation between SFR sub(IR) and SFR sub(opt) has a dispersion of 60.2 dex (semi-interquartile range). Comparison of the A sub(V) dependence on redshift and L sub(IR) reveals that for a fixed L sub(IR), there is no significant A sub(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 sub(IR) dependence.