Obscured AGN at 1.5 < z < 3.0 from the zCOSMOS-deep Survey I. Properties
  of the emitting gas in the Narrow Line Region
  We select a sample of 90 obscured (type2) AGN with 1.45<z<3.05 from the
zCOSMOS-deep galaxy sample by 5 sigma-detection of the high-ionization CIV
{\lambda}1549 narrow emission line. The presence of this feature in a galaxy
spectrum is often associated with nuclear activity, and the selection
effectiveness has been also confirmed by ultraviolet (UV) emission line ratio
diagnostic diagrams. Applying the same selection technique, a sample of 102
unobscured (type 1) AGN was collected. Taking advantage of the large amount of
multi-band data available in the COSMOS field, we investigate the properties of
the CIV-selected type 2 AGN, focusing on their host galaxies, X-ray emission
and UV emission lines. Finally, we investigate the physical properties of the
ionized gas in the Narrow Line Region (NLR) of this type 2 AGN sample,
combining the analysis of strong UV emission lines with predictions from
photo-ionization models. We find that, in order to successfully reproduce the
relative intensity of UV emission lines of the selected high-z type 2 AGN, two
new ingredients in the photo-ionization models are fundamental,i.e. small inner
radii of the NLR (~90pc for LAGN = 10^45erg/s) and the internal dissipative
micro-turbulence of the gas emitting clouds (with vmicr~100km/s). With these
modified models, we compute the gas-phase metallicity of the NLR, and our
measurements indicate a statistically significant evolution of the metal
content with redshift. Finally, we do not observe, in our CIV-selected type 2
AGN sample, a strong relationship between the NLR gas metallicity and the
stellar mass of the host galaxy.
