Star Formation Rates and Extinction Properties of IR‐luminous Galaxies in theSpitzerFirst Look Survey

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; 10 9 < 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 -1 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 -1 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 function [ A = 0.75 log( L IR / L ☉ ) - 6.35 mag] that we use to extinction correct our line luminosities. The resulting correlation between SFR IR and SFR opt has a dispersion of ~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.