A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2

Observations with the Spitzer Space Telescope have recently revealed a significant population of high-redshift (z ∼ 2) dust-obscured galaxies (DOGs) with large (rest-frame) 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 24µm to optical R-band flux density. We apply this method to observations of the ≈ 8.6 deg2 Bo¨otes Field of the NOAO Deep Wide-Field Survey, and uncover ≈2,600 DOG candidates (i.e., a surface density of 0.089 arcmin −2 ) with 24µm flux densities F24µm ≥ 0.3mJy and (R − [24]) ≥ 14 (i.e., Fν(24µm)/Fν(R) \u3e∼ 1000). These galaxies have no counterparts in the local universe. They become a larger fraction of the population at fainter 24µm flux densities, increasing from 7±0.6% of sources at F24µm ≥ 1 mJy to ≈ 13 ± 1% of the population at ≈ 0.3 mJy. These galaxies exhibit evidence of both star-formation and AGN activity, with the brighter 24µm sources being more AGN-dominated. Their mid-infrared spectral energy distributions range from power-laws (likely AGN-dominated at mid-IR wavelengths) to systems showing a “bump”, the latter likely resulting from the redshifted 1.6µm peak characteristic of most stellar populations. Using primarily the W. M. Keck Observatory and Spitzer, we have obtained spectroscopic redshifts for 86 objects within this sample, and find a broad redshift distribution which can be modeled as a Gaussian centered at ¯z ≈ 1.99 ± 0.05 and σ(z) ≈ 0.45 ± 0.05. The space density of this population is ΣDOG(F24µm ≥ 0.3 mJy) = (2.82 ± 0.05) × 10−5h 3 70 Mpc−3 , similar to that of bright sub-millimeter-selected or UV-selected galaxies at comparable redshifts. These redshifts also imply very large luminosities, with a sample median νLν(8µm) ≈ 4 × 1011L⊙, implying 8µm − 1mm luminosities of LIR \u3e∼ 1012−14L⊙ for the population. The infrared luminosity density contributed by this relatively rare DOG population is log(LIR) ≈ 8.23+0.18 −0.30. This is ≈ 60+40 −15% of that contributed by z ∼ 2 ultraluminous infrared galaxies (ULIRGs, with LIR \u3e 1012L⊙), and suggests that our simple selection criterion effectively identifies a significant fraction of z ∼ 2 ULIRGs. This IRLD is also ≈ 26 ± 14% of the total contributed by all z ∼ 2 galaxies, and comparable to that contributed by the luminous UV-bright star-forming galaxy populations at z ≈ 2. We suggest that these DOGs are the progenitors of luminous (∼ 4L ∗ ) present-day galaxies and are undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between sub-millimeter-selected galaxies and less obscured quasars or galaxies