On the dust geometry in radio-loud active galactic nuclei

TL;DR

This study investigates the dust structure in radio-loud AGN using infrared observations, revealing how dust emission varies with viewing angle and testing the effectiveness of clumpy torus models.

Contribution

It provides new insights into dust geometry and emission in AGN, and evaluates the accuracy of clumpy torus models against infrared data.

Findings

Dust emission components fit by multiple blackbodies.

Stronger warm and cold dust emission when viewed closer to jet axis.

Silicate features are always in emission and redshifted.

Abstract

We use photometric and spectroscopic infrared observations obtained with the Spitzer Space Telescope of 12 radio-loud active galactic nuclei (AGN) to investigate the dust geometry. Our approach is to look at the change of the infrared spectral energy distribution (SED) and the strength of the 10 micron silicate feature with jet viewing angle. We find that (i) a combination of three or four blackbodies fits well the infrared SED; (ii) the sources viewed closer to the jet axis appear to have stronger warm (~300 - 800 K) and cold (~150 - 250 K) dust emissions relative to the hot component; and (iii) the silicate features are always in emission and strongly redshifted. We test clumpy torus models and find that (i) they approximate well the mid-infrared part of the SED, but significantly underpredict the fluxes at both near- and far-infrared wavelengths; (ii) they can constrain the dust composition (in our case to that of the standard interstellar medium); (iii) they require relatively large (~10%-20% the speed of light) redward displacements; and (iv) they give robust total mass estimates, but are insensitive to the assumed geometry.