A significant population of very luminous dust-obscured galaxies at redshift z ∼ 2

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 Bootes field, and uncover ≈2600 dust-obscured galaxy candidates [i.e., 0.089 arcmin^-2) with 24 μm flux densities F_{24 μm} ≥ 0.3 mJy and (R - [24]) > 14 (i.e., F_ν(24 μm)/F_ν(R) ≳ 1000]. These galaxies have no counterparts in the local universe. They represent 7% ± 0.6% of the 24 μm source population at F_{24 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 z̄ ≈ 1.99 ± 0.05. The space density of this population is ⊃_DOG(F_24μm ≥ 0.3 mJy) = (2.82 ± 0.05) × 10^-5h₇₀³ Mpc^-3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median νL_ν(8 μm) ≈ 4 × 10¹¹ L_⊙. The infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log(IRLD) ≈8.23_-0.30^+0.18. This is ≈60 ^-15+40% of that contributed by z ∼ 2 ultraluminous infrared galaxies (ULIRGs, with L_IR > 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.