On the dust geometry in radio-loud active galactic nuclei

Abstract

We use photometric and spectroscopic infrared observations obtained with the Spitzer Space Telescope of 12 radio-loud active galactic nuclei 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-μm 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 per cent the speed of light) redward displacements and (iv) they give robust total mass estimates, but are insensitive to the assumed geometry. © 2010 The Authors. Journal compilation © 2010 RAS.