AGN dust tori at low and high luminosities

TL;DR

This paper examines how dust distribution affects the obscuration and classification of AGN, showing that clumpy tori can explain observed luminosity-dependent properties and the receding torus model.

Contribution

It demonstrates that a clumpy dust torus's Eddington limit is higher than smooth models and explains luminosity-dependent AGN obscuration and type-2 fractions.

Findings

Clumpy dust tori have higher Eddington limits than smooth distributions.

Obscuration decreases at low AGN luminosities (~10^42 erg/s).

Luminosity-dependent covering factors align with observations.

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 smooth dust distributions, the Eddingtion 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. For low luminosities of the order of ~10^42 erg/s, the torus changes its characteristics and obscuration becomes insufficient. In the high luminosity regime, the clumpy torus can show 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.