The infrared/X-ray view of AGN

Resumen del trabajo presentado al Spanish X-ray Astronomy, celebrado en Santander del 3 al 5 de junoio de 2015.Providing a complete census of the obscured Active Galactic Nuclei (AGN) is crucial to fully understand the cosmological growth of supermassive black holes. Nevertheless, the contribution of obscured accretion to the total AGN radiative output remains uncertain. Even the most complete AGN surveys at X-ray energies >2 keV find a population of luminous obscured AGN that is by far less numerous than required to explain the cosmic X-ray background integrated energy density. Compton-thick AGN could represent 20-30 per cent of the entire AGN population but have escaped detection so far. Thus, the majority of AGN still evade our census. Mid-infrared (mid-IR) surveys are much less affected by extinction and thus, can potentially trace the elusive obscured accretion. In this talk I will discuss recent progress on the identification of AGN in the mid-IR using the sensitive all-sky infrared survey conducted with the Wide-field Infrared Survey Explorer. I will also present the results of a recent study where, based on a detailed analysis of the X-ray absorption and mid-IR continuum properties of a complete sample of 255 AGN, we have determined the properties of the nuclear dusty toroidal structure in type 1 and type 2 AGN (the geometrical covering factor) and their dependence (or lack of it) on the AGN luminosity and line-of-sight X-ray column density. Finally I will discuss the implications of the results of our study in the context of AGN Unification Schemes.N