AGN types and unification model
  The motivation of the "unified model" is to explain the main properties of
the large zoo of active galactic nuclei with a single physical object. The
discovery of broad permitted lines in the polarized spectrum of type 2 Seyfert
galaxies in the mid 80's led to the idea of an obscuring torus, whose
orientation with respect to our line of sight was the reason of the different
optical spectra. However, after many years of observations with different
techniques, including IR and mm interferometry, the resulting properties of the
observed dust structures differ from the torus model that would be needed to
explain the type 1 vs type 2 dichotomy. Moreover, in the last years,
multi-frequency monitoring of active galactic nuclei has shown an increasing
number of transitions from one type to the other one, which cannot be explained
in terms of the simple orientation of the dusty structure surrounding the
active galactic nucleus (AGN). The interrelations between the AGN and the host
galaxy, as also shown in the Magorrian relation, suggest that the evolution of
the host galaxy may also have an important role in the observed manifestation
of the nuclei. As an example, the observed delay between the maximum star
formation activity and the onset of the AGN activity, and the higher occurrence
of type 2 nuclei in star forming galaxies, have suggested the possible
evolutionary path from, e.g., HII $\rightarrow$ AGN2 $\rightarrow$ AGN1. In the
next years the models of unification need to also consider this observational
framework and not only simple orientation effects.
