A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2
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 deg2 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 μ m >= 0.3 mJy and (R - [ 24]) >= 14 (i.e., Fν(24 μ m)/Fν(R) gtrsim 1000]. These galaxies have no counterparts in the local universe. They represent 7% +/- 0.6% of the 24 μm source population at F24 μ m >= 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 \overline{z}≈ 1.99+/- 0.05. The space density of this population is ΣDOG(F24μ m >= 0.3 mJy) = (2.82 +/- 0.05) × 10-5h370 Mpc -3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median ν Lν(8 μ m) ≈ 4 × 1011 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 > 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.
- Publication:
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The Astrophysical Journal
- Pub Date:
- April 2008
- DOI:
- 10.1086/529516
- arXiv:
- arXiv:0801.1860
- Bibcode:
- 2008ApJ...677..943D
- Keywords:
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- galaxies: formation;
- galaxies: high-redshift;
- galaxies: starburst;
- Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal