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Uncovering obscured luminous AGN with WISE

Mateos et al. (2012) presented a highly reliable and efficient mid-infrared (MIR) colour-based selection technique for luminous active galactic nuclei (AGN) using the Wide-field Infrared Survey Explorer (WISE) survey. Here we evaluate the effectiveness of this technique to identify obscured AGN missed in X-ray surveys. To do so we study the WISE properties of AGN independently selected in hard X-ray and optical surveys. We use the largest catalogue of 887 [O III]λ5007-selected type 2 quasars (QSO2s) at z<0.83 in the literature from the Sloan Digital Sky Survey, and the 258 hard (>4.5 keV) X-ray-selected AGN from the Bright Ultrahard XMM-Newton Survey (BUXS). The fraction of SDSS QSO2s in our infrared AGN selection region (wedge) increases with the AGN luminosity, reaching 66.1+4.5_4.7% at the highest [O III] luminosities in the sample. This fraction is substantially lower than for the BUXS type 1 AGN (96.1+3.0_6.3%), but consistent, within the uncertainties, with that for the BUXS type 2 AGN (75.0+14.1_19.1%) with the same luminosity. The SDSS QSO2s appear to reside in more luminous (massive) hosts than the BUXS AGN, due to the tight magnitude limits applied in the SDSS spectroscopic target selection. Since host galaxy dilution can reduce substantially the effectiveness of MIR-based techniques, this may explain the lower fraction of SDSS QSO2s in the WISE AGN wedge. The fraction of SDSS QSO2s identified as Compton-thick candidates that fall in the wedge is consistent with the fraction of all SDSS QSO2s in that zone. At the AGN luminosities involved in the comparison, Compton-thick and Compton-thin SDSS QSO2s have similar WISE colour distributions. We conclude that at high luminosities and z<1 our MIR technique is very effective at identifying both Compton-thin and Compton-thick AGN.