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Advances in Astronomy
Volume 2012 (2012), Article ID 782030, 17 pages
http://dx.doi.org/10.1155/2012/782030
Review Article

AGN Obscuration and the Unified Model

1Dipartimento di Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
2INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
3Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
4INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, Firenze, Italy
5High Energy Astrophysics Division, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 0218, USA

Received 31 August 2011; Accepted 29 December 2011

Academic Editor: Isabelle Gavignaud

Copyright © 2012 Stefano Bianchi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.