3 research outputs found
AGN Dusty Tori: II. Observational Implications of Clumpiness
From extensive radiative transfer calculations we find that clumpy torus
models with \No \about 5--15 dusty clouds along radial equatorial rays
successfully explain AGN infrared observations. The dust has standard Galactic
composition, with individual cloud optical depth \tV \about 30--100 at visual.
The models naturally explain the observed behavior of the 10\mic silicate
feature, 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 be
reproduced 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 or . Compact tori can
explain all observations, in agreement with the recent interferometric evidence
that the ratio of the torus outer to inner radius is perhaps as small as \about
5--10. Clumpy torus models can produce nearly isotropic IR emission together
with highly anisotropic obscuration, as required by observations. In contrast
with strict variants of unification schemes where the viewing-angle uniquely
determines the classification of an AGN into type 1 or 2, clumpiness implies
that it is only a probabilistic effect; a source can display type 1 properties
even from directions close to the equatorial plane. The fraction of obscured
sources depends not only on the torus angular thickness but also on the cloud
number \No. The observed decrease of this fraction at increasing luminosity can
be 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\mic
emission feature in QSO2.Comment: To appear in ApJ September 20, 200
AGN Dusty Tori: I. Handling of Clumpy Media
According to unified schemes of Active Galactic Nuclei (AGN), the central
engine is surrounded by dusty, optically thick clouds in a toroidal structure.
We have recently developed a formalism that for the first time takes proper
account of the clumpy nature of the AGN torus. We now provide a detailed report
of our findings in a two-paper series. Here we present our general formalism
for radiative transfer in clumpy media and construct its building blocks for
the AGN problem -- the source functions of individual dusty clouds heated by
the AGN radiation field. We show that a fundamental difference from smooth
density distributions is that in a clumpy medium, a large range of dust
temperatures coexist at the same distance from the radiation central source.
This distinct property explains the low dust temperatures found close to the
nucleus of NGC1068 in 10 \mic interferometric observations. We find that
irrespective of the overall geometry, a clumpy dust distribution shows only
moderate variation in its spectral energy distribution, and the 10\mic\
absorption feature is never deep. Furthermore, the X-ray attenuating column
density is widely scattered around the column density that characterizes the IR
emission. All of these properties are characteristic of AGN observations. The
assembly of clouds into AGN tori and comparison with observations is presented
in the companion paper.Comment: To appear in ApJ September 20, 200