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The obscured accretion phase in BH growth is a key ingredient in many modelslinking the AGN activity with the evolution of their host galaxy. At present, acomplete census of obscured AGN is still missing. The purpose of this work isto assess the reliability of the [NeV] emission line at 3426 A to pick upobscured AGN up to z~1 by assuming that [NeV] is a reliable proxy of theintrinsic AGN luminosity and using moderately deep X-ray data to characterizethe amount of obscuration. A sample of 69 narrow-line (Type 2) AGN atz=0.65-1.20 were selected from the 20k-zCOSMOS Bright galaxy sample on thebasis of the presence of the [NeV] emission. The X-ray properties of thesegalaxies were then derived using the Chandra-COSMOS coverage of the field; theX-ray-to-[NeV] flux ratio, coupled with X-ray spectral and stacking analyses,was then used to infer whether Compton-thin or Compton-thick absorption werepresent in these sources. Then the [NeV] luminosity function was computed toestimate the space density of Compton-thick (CT) AGN at z~0.8. Twenty-threesources were detected by Chandra, and their properties are consistent withmoderate obscuration (on average, ~a few 10^{22} cm^-2). The X-ray propertiesof the remaining 46 X-ray undetected Type 2 AGN were derived using X-raystacking analysis. Current data indicate that a fraction as high as ~40% of thepresent sample is likely to be CT. The space density of CT AGN withlogL_2-10keV>43.5 at z=0.83 is (9.1+/-2.1) 10^{-6} Mpc^{-3}, in good agreementwith both XRB model expectations and the previously measured space density forobjects in a similar redshift and luminosity range. We regard our selectiontechnique for CT AGN as clean but not complete, since even a mild extinction inthe NLR can suppress [NeV] emission. Therefore, our estimate of their spacedensity should be considered as a lower limit.

The space density of Compton-thick AGN at z~0.8 in the zCOSMOS-Bright Survey