2001A&A...365...37M

In a companion paper, Maiolino et al. (2001A&A...365...28M) presented various observational evidences for ``anomalous'' dust properties in the circumnuclear region of AGNs and, in particular, the reduced E_B–V/N_H and A_V/N_H ratios, the absence of the silicate absorption feature in mid-IR spectra of Sy2s and the absence of the carbon dip in UV spectra of reddened Sy1s. In this paper we discuss various explanations for these facts. The observational constraints favor a scenario where coagulation, catalyzed by the high densities in the circumnuclear region, yields to the formation of large grains. The resulting extinction curve is featureless, flatter than Galactic and the E_B–V/N_H and A_V/N_H ratios are significantly reduced. These results should warn about an inappropriate use of the standard Galactic extinction curve and A_V/N_H ratio when dealing with the extreme gas conditions typical of the circumnuclear clouds of AGNs. We also investigated alternative scenarios for the observed anomalous properties of dust in AGNs. Some of these scenarios might explain some of the observed properties for a few objects, but they generally fail to account for all of the observational constraints obtained for the large sample of AGNs studied in these works.