A Significant Population of Very Luminous Dust‐Obscured Galaxies at Redshiftz∼ 2

Abstract

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 deg^2 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 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^−5^h3 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 × 10^11 L⊙. The 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 > 10^12 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.