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The Spitzer Space Telescope has revealed a significant population of high-redshift (z ~ 2) dust-obscured galaxies with large mid-infrared to ultraviolet luminosity ratios. Due to their optical faintness, these galaxies have been previously missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-redshift population based solely on the ratio of the observed mid-infrared 24 μm to optical R-band flux density. We apply this method to observations of the ≈8. 6 deg² NOAO Deep Wide-Field Survey Boötes field, and uncover ≈2600 dust-obscured galaxy candidates i. e. , 0. 089 arcmin^−2) with 24 μm flux densities F24 μm ≥ 0. 3 mJy and (R − [24) ≥ 14 (i. e. , Fν (24 μm) /Fν (R) ≳1000]. These galaxies have no counterparts in the local universe. They represent 7% ± 0. 6% of the 24 μm source population at F24 μm ≥ 1 mJy but increase to ≈13% ± 1% of the population at ≈0. 3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 μm sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered at z ≈ 1. 99±0. 05. The space density of this population is ΣDOG (F24μ m ≥ 0. 3 mJy) = (2. 82 ± 0. 05) × 10^−5ʰ3 70 Mpc^−3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median νLν (8 μm) ≈4 × 10¹1 L⊙. infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log (IRLD) ≈8. 23^+0. 18 −0. 30. This is ≈60^+40 −15% of that contributed by z ~ 2 ultraluminous infrared galaxies (ULIRGs, with LIR > 10¹2 L⊙) ; our simple selection thus identifies a significant fraction of z ~ 2 ULIRGs. This IRLD is ≈26% ± 14% of the total contributed by all z ~ 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous (~4L*) present-day galaxies, seen undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between submillimeter-selected galaxies and less obscured quasars or galaxies.
Dey et al. (Fri,) studied this question.