Observations with the Spitzer Space Telescope have recently revealed a significant population of high-redshift ( z \sim 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 24 \mu m to optical R -band flux density .
We apply this method to observations of the \approx 8.6 ~ { } { deg } ^ { 2 } Boötes Field of the NOAO Deep Wide-Field Survey , and uncover \approx 2,600 DOG candidates ( i.e. , a surface density of 0.089 arcmin ^ { -2 } ) with 24 \mu m flux densities F _ { 24 \mu m } \geq 0.3 mJy and ( R - [ 24 ] ) \geq 14 ( i.e. , F _ { \nu } ( { 24 \mu m } ) / F _ { \nu } ( R ) \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0. % 86 pt } { $ \textstyle > $ } } { \sim } $ } 1000 ) .
These galaxies have no counterparts in the local universe .
They become a larger fraction of the population at fainter 24 \mu m flux densities , increasing from 7 \pm 0.6 % of sources at F _ { 24 \mu m } \geq 1 mJy to \approx 13 \pm 1 % of the population at \approx 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 .
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 1.6 \mu m 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 \bar { z } \approx 1.99 \pm 0.05 and \sigma ( z ) \approx 0.45 \pm 0.05 .
The space density of this population is \Sigma _ { DOG } ( F _ { 24 \mu m } \geq 0.3 ~ { } { mJy } ) = ( 2.82 \pm 0.05 ) \times 10 ^ % { -5 } h _ { 70 } ^ { 3 } ~ { } { 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 \nu L _ { \nu } ( 8 \mu { m } ) \approx 4 \times 10 ^ { 11 } L _ { \odot } , implying { 8 \mu m - 1 mm } luminosities of L _ { IR } \raisebox { -2.58 pt } { $ \stackrel { \raisebox { -0.86 pt } { $ \textstyle > $ } } { % \sim } $ } 10 ^ { 12 - 14 } L _ { \odot } for the population .
The infrared luminosity density contributed by this relatively rare DOG population is { log } ( L _ { IR } ) \approx 8.23 ^ { +0.18 } _ { -0.30 } .
This is \approx 60 ^ { +40 } _ { -15 } % of that contributed by z \sim 2 ultraluminous infrared galaxies ( ULIRGs , with L _ { IR } > 10 ^ { 12 } L _ { \odot } ) , and suggests that our simple selection criterion effectively identifies a significant fraction of z \sim 2 ULIRGs .
This IRLD is also \approx 26 \pm 14 % of the total contributed by all z \sim 2 galaxies , and comparable to that contributed by the luminous UV-bright star-forming galaxy populations at z \approx 2 .
We suggest that these DOGs are the progenitors of luminous ( \sim 4 L ^ { * } ) 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 .