Obscured AGN at z~1 from the zCOSMOS-Bright Survey I. Selection and
  Optical Properties of a [Ne v]-selected sample
  A sample of 94 narrow line AGN with 0.65<z<1.20 has been selected from the
20k-Bright zCOSMOS galaxy sample by detection of the high-ionization [NeV]3426
line. Taking advantage of the large amount of data available in the COSMOS
field, the properties of the [NeV]-selected Type-2 AGN have been investigated,
focusing on their host galaxies, X-ray emission, and optical line flux ratios.
Finally, the diagnostic developed by Gilli et al. (2010), based on the X-ray to
[NeV] luminosity ratio, has been exploited to search for the more heavily
obscured AGN. We found that [Ne v]-selected narrow line AGN have Seyfert 2-like
optical spectra, although with emission line ratios diluted by a star-forming
component. The ACS morphologies and stellar component in the optical spectra
indicate a preference for our Type-2 AGN to be hosted in early-spirals with
stellar masses greater than 10^(9.5-10)Msun, on average higher than those of
the galaxy parent sample. The fraction of galaxies hosting [NeV]-selected
obscured AGN increases with the stellar mass, reaching a maximum of about 3% at
2x10^11 Msun. A comparison with other selection techniques at z~1 shows that
the detection of the [Ne v] line is an effective method to select AGN in the
optical band, in particular the most heavily obscured ones, but can not provide
by itself a complete census of AGN2. Finally, the high fraction of
[NeV]-selected Type-2 AGN not detected in medium-deep Chandra observations
(67%) is suggestive of the inclusion of Compton-thick sources in our sample.
The presence of a population of heavily obscured AGN is corroborated by the
X-ray to [NeV] ratio; we estimated, by mean of X-ray stacking technique and
simulations, that the Compton-thick fraction in our sample of Type-2 AGN is
43+-4%, in good agreement with standard assumptions by the XRB synthesis
models.
