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 mu m, 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 mu m (W1) and least sensitive at 22 mu m (W4). The W1 band probes massive early-type galaxies out to z greater than or similar to 1. This is more distant than SDSS identified early-type galaxies, consistent with the fact that 28% of 3.4 mu m sources have faint or no r-band counterparts (r > 22.2). In contrast, 92%-95% of 12 mu m and 22 mu m sources have SDSS optical counterparts with r <= 22.2. WISE 3.4 mu m detects 89.8% of the entire SDSS QSO catalog at S/N-W1 > 7 sigma, but only 18.9% at 22 mu m with S/N-W4 > 5 sigma. 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 less than or similar to 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 similar to 2 with extremely red colors, r - W4 > 14 or well-detected 22 mu m sources lacking detections in the 3.4 and 4.6 mu m bands. The surface density of z similar to 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.