Characterizing the Mid-Infrared Extragalactic Sky with WISE and SDSS

The Wide-field Infrared Survey Explorer (WISE) has completed its all-sky survey in four channels at 3.4-22 μm, detecting hundreds of millions of objects. We merge the WISE mid-infrared data with optical data from the Sloan Digital Sky Survey (SDSS) and provide a phenomenological characterization of WISE extragalactic sources. WISE is most sensitive at 3.4 μm (W1) and least sensitive at 22 μm (W4). The W1 band probes massive early-type galaxies out to z 1. This is more distant than SDSS identified early-type galaxies, consistent with the fact that 28% of 3.4 μm sources have faint or no r-band counterparts (r > 22.2). In contrast, 92%-95% of 12 μm and 22 μm sources have SDSS optical counterparts with r ≤ 22.2. WISE 3.4 μm detects 89.8% of the entire SDSS QSO catalog at S/N W1 >7σ, but only 18.9% at 22 μm with S/N W4 > 5σ. We show that WISE colors alone are effective in isolating stars (or local early-type galaxies), star-forming galaxies, and strong active galactic nuclei (AGNs)/QSOs at z 3. We highlight three major applications of WISE colors: (1) Selection of strong AGNs/QSOs at z ≤ 3 using W1 – W2 > 0.8 and W2 < 15.2 criteria, producing a better census of this population. The surface density of these strong AGN/QSO candidates is 67.5 ± 0.14 deg–2. (2) Selection of dust-obscured, type-2 AGN/QSO candidates. We show that WISE W1 – W2 > 0.8, W2 < 15.2 combined with r – W2 > 6 (Vega) colors can be used to identify type-2 AGN candidates. The fraction of these type-2 AGN candidates is one-third of all WISE color-selected AGNs. (3) Selection of ultraluminous infrared galaxies (ULIRGs) at z ~ 2 with extremely red colors, r– W4 > 14 or well-detected 22 μm sources lacking detections in the 3.4 and 4.6 μm bands. The surface density of z ~ 2 ULIRG candidates selected with r – W4 > 14 is 0.9 ± 0.07 deg–2 at S/N W4 ≥ 5 (the corresponding, lowest flux density of 2.5 mJy), which is consistent with that inferred from smaller area Spitzer surveys. Optical spectroscopy of a small number of these high-redshift ULIRG candidates confirms our selection, and reveals a possible trend that optically fainter or r – W4 redder candidates are at higher redshifts.