STAR FORMATION RATES AND EXTINCTION PROPERTIES OF IR-LUMINOUS GALAXIES IN THE SPITZER FIRST LOOK SURVEY

摘要

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)/solar luminosity < 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 solar mass yr~(-1) have extinction values consistent with normal spirals (A_V ≈ 1.0 mag). By contrast, LIRGs at z approx > 1, which comprise a fraction of our sample, have SFR ≈ 100 solar mass 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_V~(IR) function [A_V~(IR) = 0.75 log (L_(IR)/solar luminosity) - 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.