In a companion paper, Maiolino et al. (2000) 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 Av/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 Av/N_H ratios are significantly reduced. These
results should warn about an unappropriate use of the standard Galactic
extinction curve and Av/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.Comment: 13 pages, 7 figures, accepted for publication in A&
