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

Abstract

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 105.5 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 [OIII]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 ([OIII]/Hβ versus [N ii]/Hα and [OIII]/Hβ versus [S ii]/Hα) to isolate AGN in the redshift range 0.15 < z < 0.45 and the diagnostic diagram [OIII]/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 luminositydependent 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. © 2010 ESO.