The [O III] emission line luminosity function of optically selected type-2 AGN from zCOSMOS

Aims. We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sample covers a wide redshift range (0.15 \textless z \textless 0.92) and is deeper than any other previous study, encompassing the luminosity range 10(5.5) L-circle dot \textless L-[OIII] \textless 10(9.1) L-circle dot. We explore the intrinsic properties of these AGN and the relation to their X-ray emission (derived from the XMM-COSMOS observations). We study their evolution by computing the [OIII]lambda 5007 angstrom line luminosity function (LF) and we constrain the fraction of obscured AGN as a function of luminosity and redshift. Methods. The sample was selected on the basis of the optical emission line ratios, after applying a cut to the signal-to-noise ratio (S/N) of the relevant lines. We used the standard diagnostic diagrams ([O III]/H beta versus [N II]/H alpha and [OIII]/H beta versus [S II]/H alpha) to isolate AGN in the redshift range 0.15 \textless z \textless 0.45 and the diagnostic diagram [O III]/H beta versus [O II]/H beta to extend the selection to higher redshift (0.5 \textless z \textless 0.92). Results. Combining our sample with one drawn from SDSS, we found that the best description of the evolution of type-2 AGN is a luminosity-dependent density evolution model. Moreover, using the type-1 AGN LF we were able to constrain the fraction of type-2 AGN to the total (type-1 + type-2) AGN population. We found that the type-2 fraction decreases with luminosity, in agreement with the most recent results, and shows signs of a slight increase with redshift. However, the trend with luminosity is visible only after combining the SDSS+zCOSMOS samples. From the COSMOS data points alone, the type-2 fraction seems to be quite constant with luminosity.