The dusty heart of nearby active galaxies -- II. From clumpy torus models to physical properties of dust around AGN

TL;DR

This paper advances 3D clumpy dust torus models around AGN by improving radiative transfer handling, enabling better interpretation of IR observations and constraining dust distribution properties through combined spectral and interferometric data.

Contribution

It introduces an upgraded radiative transfer model for clumpy dust tori, incorporating diffuse radiation, and provides methods to interpret IR and interferometric observations to determine dust distribution parameters.

Findings

Models with standard ISM dust match observed IR SEDs.

Mid-IR spectral index constrains radial dust distribution.

Interferometry enhances parameter estimation accuracy.

Abstract

The dusty environments (= "dust tori'') of AGN are now in reach of observations. Following our paper I on ground-based mid-IR spectro-photometry (H\"onig et al. 2010), we present an upgrade to our radiative transfer model of 3-dimensional clumpy dust tori. The upgrade with respect to H\"onig et al. (2006) concerns an improved handling of the diffuse radiation field in the torus which is approximated by a statistical approach. The models are presented as tools to translate classical and interferometric observations into characteristic properties of the dust distribution. We compare model SEDs for different chemical and grain-size compositions of the dust and find that clouds with standard ISM dust and optical depth tau_V~50 appear in overall agreement with observed IR SEDs. By studying parameter dependencies, it is shown that type 1 AGN SEDs, in particular the mid-IR spectral index, can be used to constrain the radial dust cloud distribution power-law index 'a', while other parameters are more difficult to assess using SEDs only. Interferometry adds important additional information for modeling when interpreted simultaneously with the SED. Although type 2 AGN can, in principle, be used to constrain model parameters as well, obscuration effects make the analysis more ambiguous. We propose a simple, interferometry-based method to distinguish between "compact'' and "extended'' radial dust distributions without detailed modeling of the data and introduce a way to easily determine individual or sample average model parameters using the observed optical depth in the silicate feature and the mid-IR spectral index.