Mid-Infrared Selection of Active Galactic Nuclei With Thewide-Field Infrared Survey Explorer. I. Characterizingwise-Selected Active Galactic Nuclei in Cosmos

The Wide-field Infrared Survey Explorer (WISE) is an extremely capable and efficient black hole finder. We present a simple mid-infrared color criterion, W 1 − W 2 ⩾ 0.8 (i.e., 3.4−4.6 ⩾0.8, Vega), which identifies 61.9 ± 5.4 active galactic nucleus (AGN) candidates per deg 2 to a depth of W 2 ∼ 15.0. This implies a much larger census of luminous AGNs than found by typical wide-area surveys, attributable to the fact that mid-infrared selection identifies both unobscured (type 1) and obscured (type 2) AGNs. Optical and soft X-ray surveys alone are highly biased toward only unobscured AGNs, while this simple WISE selection likely identifies even heavily obscured, Compton-thick AGNs. Using deep, public data in the COSMOS field, we explore the properties of WISE -selected AGN candidates. At the mid-infrared depth considered, 160 μJy at 4.6 μm, this simple criterion identifies 78% of Spitzer mid-infrared AGN candidates according to the criteria of Stern et al. and the reliability is 95%. We explore the demographics, multiwavelength properties and redshift distribution of WISE -selected AGN candidates in the COSMOS field.