Mid-Infrared Selection of AGN with the Wide-Field Infrared Survey
  Explorer. I. Characterizing WISE-Selected AGN 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,
W1-W2 \geq 0.8 (i.e., [3.4]-[4.6] \geq 0.8, Vega), which identifies 61.9 \pm
5.4 AGN candidates per deg2 to a depth of W2 = 15.0. This implies a much larger
census of luminous AGN than found by typical wide-area surveys, attributable to
the fact that mid-infrared selection identifies both unobscured (type 1) and
obscured (type 2) AGN. Optical and soft X-ray surveys alone are highly biased
towards only unobscured AGN, while this simple WISE selection likely identifies
even heavily obscured, Compton-thick AGN. Using deep, public data in the COSMOS
field, we explore the properties of WISE-selected AGN candidates. At the
mid-infrared depth considered, 160 uJy at 4.6 microns, this simple criterion
identifies 78% of Spitzer mid-infrared AGN candidates according to the criteria
of Stern et al. (2005) and the reliability is 95%. We explore the demographics,
multiwavelength properties and redshift distribution of WISE-selected AGN
candidates in the COSMOS field.
