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 < z < 0.92) and is deeper than any other previous study, encompassing the luminosity range 10^5.5 L_⊙ < L_[OIII] < 10^9.1 L_⊙. 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 [O iii]λ5007 Å 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]/Hβ versus [N ii]/Hα and [O iii]/Hβ versus [S ii]/Hα) to isolate AGN in the redshift range 0.15 < z < 0.45 and the diagnostic diagram [O iii]/Hβ versus [O ii]/Hβ to extend the selection to higher redshift (0.5 < z < 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.

Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, program 175.A-0839.Appendix A is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/510/A56