AGN Dusty Tori. II. Observational Implications of Clumpiness

From extensive radiative transfer calculations we find that clumpy torusmodels with \No \about 5--15 dusty clouds along radial equatorial rayssuccessfully explain AGN infrared observations. The dust has standard Galacticcomposition, with individual cloud optical depth \tV \about 30--100 at visual.The models naturally explain the observed behavior of the 10\mic silicatefeature, in particular the lack of deep absorption features in AGN of any type.The weak 10\mic emission feature tentatively detected in type 2 QSO can bereproduced if in these sources \No drops to \about 2 or \tV exceeds \about 100.The clouds angular distribution must have a soft-edge, e.g., Gaussian profile,the radial distribution should decrease as $1/r$ or $1/r^2$. Compact tori canexplain all observations, in agreement with the recent interferometric evidencethat the ratio of the torus outer to inner radius is perhaps as small as \about5--10. Clumpy torus models can produce nearly isotropic IR emission togetherwith highly anisotropic obscuration, as required by observations. In contrastwith strict variants of unification schemes where the viewing-angle uniquelydetermines the classification of an AGN into type 1 or 2, clumpiness impliesthat it is only a probabilistic effect; a source can display type 1 propertieseven from directions close to the equatorial plane. The fraction of obscuredsources depends not only on the torus angular thickness but also on the cloudnumber \No. The observed decrease of this fraction at increasing luminosity canbe explained with a decrease of either torus angular thickness or cloud number,but only the latter option explains also the possible emergence of a 10\micemission feature in QSO2.Comment: To appear in ApJ September 20, 200