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 similar to 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 mu m to optical R-band flux density. We apply this method to observations of the approximate to 8.6 deg(2) NOAO Deep Wide-Field Survey Bootes field, and uncover approximate to 2600 dust-obscured galaxy candidates [i. e., 0.089 arcmin(-2)) with 24 mu m flux densities F-24 mu m >= 0.3 mJy and (R-[24]) >= 14 (i. e., F-v(24 mu m)/ F-v(R) >= 1000]. These galaxies have no counterparts in the local universe. They represent 7% +/-0: 6% of the 24 mu m source population at F-24 mu m >= 1 mJy but increase to approximate to 13% +/-1% of the population at approximate to 0.3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 mu m sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered at (z) over bar approximate to 1.99 +/-0.05. The space density of this population is Sigma(DOG) (F-24 mu m >= 0.3 mJy) (2.82 +/-0.05) x 10(-5)h(70)(3) Mpc(-3), similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median VLv(8 mu m) approximate to 4 x 10(11) L-circle dot. The infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log(IRLD) approximate to 8: 23(-0.30)(+0.18). This is approximate to 60(-15)(+40)% of that contributed by z similar to 2 ultraluminous infrared galaxies (ULIRGs, with L-IR > 10(12) L-circle dot); our simple selection thus identifies a significant fraction of z similar to 2 ULIRGs. This IRLD is approximate to 26% +/-14% of the total contributed by all z similar to 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous (similar to 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.