CHARACTERIZING THE MID-INFRARED EXTRAGALACTIC SKY WITH<i>WISE</i>AND SDSS

Abstract

The Wide-field Infrared Survey Explorer (WISE) has completed its all-sky survey in four channels at 3.4-22 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 m (W 1) and least sensitive at 22 m (W 4). The W 1 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 m sources have faint or no r-band counterparts (r > 22.2). In contrast, 92%-95% of 12 m and 22 m sources have SDSS optical counterparts with r 22.2. WISE 3.4 m detects 89.8% of the entire SDSS QSO catalog at S/N W 1 >7 , but only 18.9% at 22 m with S/N W 4 > 5 . 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 W 1 -W 2 > 0.8 and W 2 < 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 . ( We show that WISE W 1 -W 2 > 0.8, W 2 < 15.2 combined with r -W 2 > 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.