Obscured active galactic nuclei

I discuss some of the issues related to the obscuration in Active Galactic Nuclei (A GNs) and propose some models which could account for some of the properties observed in this class of objects. I discuss the obscuration of A GNs at high luminosities and show that in QSOs the covering factor of the obscuring dusty medium is reduced with respect to Seyfert nuclei. This effect may be the result of the interaction of dust grains, accelerated by the strong radiation pressure, with the gas and with other grains which is likely to cause the disruption of dust. At higher redshift the fraction of obscured A GNs is likely to be higher, as a consequence of the higher rate of interactions and distorted morphologies which drive gas into the nuclear region. To this regard, I discuss some evidences for a connection between non-axisymmetric potentials in the host galaxy and nuclear obscuration in local Sy2s. I discuss the case of "elusive" AGNs hosted in starburst galaxies which are not identified by means of the classical optical tracers. These are probably AGNs which are completely buried, i.e. obscured in all directions. I show that high angular resolution mid-IR observations may be a powerful tool to identify this class of objects. Finally, I tackle the hotly debated issue of the relative role of AGN and starburst in the budget of the bolometric luminosity. By combining sub-mm and mid-IR images I show that some nearby Sy2s are clearly dominated by the circumnuclear star formation. Some candidate type 2 QSOs have properties similar to starburst-dominated Sy2s, suggesting that also the former are mostly powered by starburst activity.