2006ApJ...637..227C

We investigate the instantaneous star formation rates (SFRs) and extinction properties for a large (N=274), near-infrared (NIR: 2.2 µm) + mid-infrared (MIR: 24 µm)-selected sample of normal to ultraluminous infrared galaxies (ULIRGs; 10⁹<L_IR/L_☉<10^12.5) with <z≳0.8 in the Spitzer Extragalactic First Look Survey (FLS). We combine 24 µm observations with high-resolution Keck DEIMOS spectroscopy to derive optical emission-line (Hα, Hβ, and [O II]) and infrared star formation rates (SFR_opt and SFR_IR, 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 (~12% of the sample). Objects with SFRs of a few M_☉/yr have extinction values consistent with normal spirals (A_V~1.0 mag). By contrast, LIRGs at z≳1, which comprise a fraction of our sample, have SFR~100 M_☉/yr and a mean A_V~2.5 mag. This translates to a 97% mean [O II] λλ3727 attentuation and in extreme cases is as high as 99.7%. We derive an IR-luminosity-dependent A^IR_Vfunction [A^IR_V=0.75log(L_IR/L_☉)-6.35 mag] that we use to extinction correct our line luminosities. The resulting correlation between SFR_IRand SFR_opt has a dispersion of ∼0.2 dex (semi-interquartile range). Comparison of the A_Vdependence on redshift and L_IRreveals that for a fixed L_IR, there is no significant A_Vevolution. 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_IRdependence.