Publications of the MPIfR
Optical & Infrared
Interferometry Group
Hönig, S.F. and Beckert, T.
AGN dust tori at low and high luminosities
Monthly Notices of the Royal Astronomical Society,
v.380, pg.1172-1176 (2007)
Abstract
A cornerstone of AGN unification schemes is the presence of an
optically and geometrically thick dust torus. It provides the
obscuration to explain the difference between type 1 and type 2 AGN.
We investigate the influence of the dust distribution on the Eddington
limit of the torus. For homogeneous dust distributions, the Eddington
limit on the dust alone is 5 orders of magnitudes
below the limit for electron scattering in a fully ionized plasma,
while a clumpy dust torus has an Eddington limit slightly larger than
the classical one. We study the behaviour of a clumpy torus
at low and high AGN luminosities. Fro lwo luminosities below ~ 1042
erg s-1, the torus changes its characteristics and
obscuration becomes sufficient. In the QSO regime, the clumpy torus
shows a
behaviour which is consistent with the "receding torus" picture. The
derived luminosity-dependent fraction of type-2-objects agrees with
recent observational results. Moreover, the luminosity-
dependent covering factor in a clumpy torus may explain the presence of
broad-line AGN with high column densities in X-rays.
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