2008ApJ...677..943D

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/arcmin2) with 24 µm flux densities F_24µm≥0.3 mJy and (R-[24])≥14 (i.e., F_ν(24µm)/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_24µm≥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{d1}~1.99±0.05. The space density of this population is Σ_DOG(F_24µm≥0.3 mJy)=(2.82±0.05)x10^–5h³₇₀/Mpc3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median νL_ν(8 µm)~4x10¹¹ 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 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.