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Published April 20, 2008 | Published
Journal Article Open

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 ~ 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⊙. 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.

Additional Information

© 2008 The American Astronomical Society. Received 2007 November 6; accepted 2008 January 22. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. We are grateful to the expert assistance of the staff of Kitt Peak National Observatory where the Boötes field observations of the NDWFS were obtained. The authors thank NOAO for supporting the NOAO Deep Wide-Field Survey. In particular, we thank Jenna Claver, Lindsey Davis, Alyson Ford, Emma Hogan, Tod Lauer, Lissa Miller, Erin Ryan, Glenn Tiede, and Frank Valdes for their able assistance with the NDWFS data. AD thanks Naveen Reddy, Greg Rudnick, Mark Dickinson, and Samir Salim for illuminating discussions about the innumerable z ≈ 2 galaxy populations and local UV-bright populations, and Naveen Reddy, Alex Pope, Casey Papovich and the anonymous referee for constructive comments on the manuscript. A. D. is grateful to the Institute for Astronomy of the University of Hawaii, the Spitzer Science Center, the University of California at Santa Cruz, and Steward Observatory for their hospitality during his sabbatical year, and for enabling the writing of this paper. The research activities of A. D. and B. T. J. are supported by NOAO, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Support for E. Le Floc'h was provided by NASA through the Spitzer Space Telescope Fellowship Program. We also thank the staffs of the W. M. Keck Observatory and the Gemini- North Observatory, where some of the galaxy redshifts were obtained. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and SECYT(Argentina). The authors also wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.

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