Shaking the grounds of unification: are type 1 and type 2 AGN intrinsically different?

The simplest standard unified models of Active Galactic Nuclei (AGN), put forward more than 30 years ago, postulate that the diversity of observed properties of AGN can be largely explained as a viewing angle effect resulting in anisotropic nuclear obscuration. A key ingredient of these models is a homogeneous distribution of dust and gas located at tens of parsecs that obscures the AGN nuclear region from certain lines-of-sight (the ‘torus’). Recent observational constraints, especially in the mid-infrared regime, are questioning the validity of the simplest version of these models.

To confirm or reject on firm statistical grounds the basic premises of standard AGN unification we have conducted a study aimed to determine the distributions of geometrical covering factors of AGN dusty tori. To do so we have used a large, uniformly-selected, complete AGN sample and one of the most up to date radiative transfer codes available in the literature to model the infrared emission of AGN tori. I will present the main results of this study where we demonstrate that, contrary to what is postulated by standard unification, type 1 and type 2 AGN are intrinsically different. Furthermore, we find that not only the viewing angle and AGN luminosity, but also the covering factor of the torus, determines the optical and infrared appearance of an AGN.