Radiative transfer modelling of parsec-scale dusty warped discs

TL;DR

This study uses dust radiative transfer simulations to investigate infrared signatures of parsec-scale warped dusty discs in active galactic nuclei, revealing their potential to explain observed dust morphologies and spectral features.

Contribution

First to model warped dusty discs with radiative transfer, explaining their infrared signatures and matching observations like in the Circinus galaxy.

Findings

Warped discs show distinct infrared brightness variations based on orientation.

Silicate features vary from absorption to emission depending on viewing angle.

Warped disc models can explain observed dust morphology without changing dust composition.

Abstract

Warped discs have been found on (sub-)parsec scale in some nearby Seyfert nuclei, identified by their maser emission. Using dust radiative transfer simulations we explore their observational signatures in the infrared in order to find out whether they can partly replace the molecular torus. Strong variations of the brightness distributions are found, depending on the orientation of the warp with respect to the line of sight. Whereas images at short wavelengths typically show a disc-like and a point source component, the warp itself only becomes visible at far-infrared wavelengths. A similar variety is visible in the shapes of the spectral energy distributions. Especially for close to edge-on views, the models show silicate feature strengths ranging from deep absorption to strong emission for variations of the lines of sight towards the warp. To test the applicability of our model, we use the case of the Circinus galaxy, where infrared interferometry has revealed a highly elongated emission component matching a warped maser disc in orientation and size. Our model is for the first time able to present a physical explanation for the observed dust morphology as coming from the AGN heated dust. As opposed to available torus models, a warped disc morphology produces a variety of silicate feature shapes for grazing lines of sight, close to an edge-on view. This could be an attractive alternative to a claimed change of the dust composition for the case of the nearby Seyfert 2 galaxy NGC 1068, which harbours a warped maser disc as well.