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

The Astrophysical Journal2008Vol. 677(2), pp. 943–956

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Arjun Dey, B. T. Soifer, Vandana Desai, Kate Brand, E. Le Floc’h, M. J. I. Brown, Buell T. Jannuzi, L. Armus, Shane Bussmann, M. Brodwin, Chao Bian, Peter Eisenhardt, S. J. U. Higdon, D. W. Weedman, S. P. Willner

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 Boötes 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⊙.\nThe 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.\n

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Abstract

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