%0 Journal Article
%T AGN Obscuration and the Unified Model
%A Stefano Bianchi
%A Roberto Maiolino
%A Guido Risaliti
%J Advances in Astronomy
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/782030
%X Unification Models of Active Galactic Nuclei postulate that all the observed differences between type 1 and type 2 objects are due to orientation effects with respect to the line of sight to the observer. The key ingredient of these models is the obscuring medium, historically envisaged as a toroidal structure on a parsec scale. However, many results obtained in the last few years are clearly showing the need for a more complex geometrical distribution of the absorbing media. In this paper, we review the various pieces of evidence for obscuring media on different scales, from the vicinity of the black hole to the host galaxy, in order to picture an updated unification scenario explaining the complex observed phenomenology. We conclude by mentioning some of the open issues. 1. Introduction: The Standard Unified Model In this paper, we discuss the recent developments on the AGN unified models, specifically for what concern the geometry, location, and physics of the absorbing medium. Before discussing the more recent results, in this section we shortly review the early, classical arguments that historically led to the formulation of the standard Unified Model. An early review of the initial results was also given in Antonucci [1]. The first unification attempts have been focussed on polarization measurements. In particular Antonucci [2] found a perpendicular alignment of optical polarization relative to the radio axis in a sample of radio galaxies, which was interpreted as due to scattering of photons, whose direction before entering the line of sight was primarily in the vertical direction. Shortly after, additional evidence was found in low luminosity, local AGN, and specifically Seyfert galaxies [3]. Seyfert 1 galaxies are characterized by the presence of broad optical permitted lines ( £¿km/s), such as H and H , that are not observed in Seyfert 2 galaxies. However, the presence of both strong high ionization and low ionization narrow ( £¿km/s) forbidden lines (such as [O£¿III], [Ne£¿III], [O£¿II], [O£¿I], [N£¿II], [S£¿II]), and several very high ionization coronal lines (such as [Fe£¿X], [Fe£¿XI], [Si£¿IX], [Si£¿X]) is common to both types of Seyfert galaxies and with similar line ratios. The latter finding suggested that all Seyfert galaxies are powered by the same intrinsic engine. A strong observational evidence of a unification between type 2 and type 1 Seyfert nuclei has been the discovery of broad optical lines in the polarized spectrum of the archetypal Seyfert 2, NGC 1068, obtained by Antonucci and Miller [4]. This finding revealed the presence of a Broad
%U http://www.hindawi.com/journals/aa/2012/782030/