Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies

Abstract

Context. Tori of Active Galactic Nuclei (AGN) are made up of a mixture of hot and cold gas, as well as dust. In order to protect the dust grains from destruction by the surrounding hot gas as well as by the energetic (UV/optical) radiation from the accretion disk, the dust is often assumed to be distributed in clouds. Aims. A new three-dimensional model of AGN dust tori is extensively investigated. The torus is modelled as a wedge-shaped disk within which dusty clouds are randomly distributed throughout the volume, by taking the dust density distribution of the corresponding continuous model into account. We especially concentrate on the differences between clumpy and continuous models in terms of the temperature distributions, the surface brightness distributions and interferometric visibilities, as well as spectral energy distributions. Methods. Radiative transfer calculations with the help of the three-dimensional Monte Carlo radiative transfer code MC3D are used in order to simulate spectral energy distributions as well as surface brightness distributions at various wavelengths. In a second step, interferometric visibilities for various inclination as well as position angles and baselines are calculated, which can be used to directly compare our models to interferometric observations with the MIDI instrument. Results. We find that the radial temperature distributions of clumpy models possess significantly enhanced scatter compared to the continuous cases. Even at large distances, clouds can be heated directly by the central accretion disk. The existence of the silicate 10 µm-feature in absorption or in emission depends sensitively on the distribution, the size and optical depth of clouds in the innermost part of the dust distribution. With this explanation, failure and success of previous modelling efforts of clumpy tori can be understood. The main reason for this outcome are shadowing effects of clouds within the central region. We underline this result with the help of several parameter variations. After adapting the parameters of our clumpy standard model to the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively explain recent mid-infrared interferometric observations performed with MIDI, as well as high resolution spectral data.

The dusty heart of nearby active galaxies

“…In particular, silicate emission features in type 1 AGN are much weaker than expected from early torus models, which used smooth dust distributions. As shown in the literature, this might be explained by clumpiness of the dust in the torus, which leads to an overall suppression of the emission feature (Nenkova et al 2002;Dullemond & van Bemmel 2005;Hönig et al 2006;Schartmann et al 2008;Nenkova et al 2008;Hönig & Kishimoto 2010). However, as will be shown in the second paper of this series (Hönig & Kishimoto 2010, pre-print available on astro-ph), weak emission features are not a generic property of clumpy torus models but might be used to constrain some model parameters of the dust distribution in clumpy tori.…”

“…Clumpy torus models based on ISM dust well reproduce the overall IR dust reemission and the silicate absorption features in detail (e.g. Hönig et al 2006Polletta et al 2008;Schartmann et al 2008). For type 1 AGN, the situation is less clear.…”

Dusty star-forming galaxies at high redshift

“…When discussing these models, it's important to keep in mind that the accuracy or applicability of these models cannot be tested with data since it simply does not exist in enough detail to disentangle effects of geometry, distribution, optical depth, etc. It's also important to note that many have done work in this area, particularly modeling radiative transfer in local starburst populations to generate SEDs (Efstathiou & Rowan-Robinson, 1995;Efstathiou et al, 2000;Efstathiou & Siebenmorgen, 2009;Nenkova et al, 2002;Dullemond & van Bemmel, 2005;Piovan et al, 2006;Nenkova et al, 2008;Takagi et al, 2003;Fritz et al, 2006;Hönig et al, 2006;Schartmann et al, 2008), but here we try to focus on the techniques which have been most commonly employed for SED fitting of high-z dusty starbursts 11 . Silva et al (1998) developed the Grasil code to model galaxy emission by explicitly accounting for dust absorption and emission from the ultraviolet through to the far-infrared.…”

Star formation inz> 1 3CR host galaxies as seen byHerschel

“…There exists broad agreement that the AGN-heated nuclear dust is mainly located in clumps which are distributed in a toroidal pattern altogether referred to as the AGN torus (e.g. Nenkova et al 2002;Kuraszkiewicz et al 2003;Hönig et al 2006;Schartmann et al 2008). To account for the emission from the AGN heated dust, we chose the library of torus models from Hönig & Kishimoto (2010).…”