Thick tori around active galactic nuclei: a comparison of model predictions with observations of the infrared continuum and silicate features

The continuum expected from active galactic nuclei (AGN) surrounded by thick tori is compared to the data available for a sample of optically selected Seyfert 1 galaxies. The optical and near-IR nuclear fluxes have previously been derived for these objects, and hence the spectral energy distributions (SEDs) of their active nuclei can be analysed. We perform detailed calculations of the 0.1 to |$1000\, \mu {\rm m}$| SEDs for AGN surrounded by dust tori with different opening angles. The dust mixture is mimicked by using three silicate and three graphite grains of different sizes. The radiative transfer equation for a cloud having azimuthal symmetry and containing a mixture of dust grains is solved by means of a numerical code that takes absorption, emission and scattering into account. We discuss the optimization of the free parameters by comparing the spectra predicted by the code to available data. The general absence of the silicate emission feature at about |$10\, \mu {\rm m}$| in the spectra of broad- line AGN and the general presence of the |$10\,\mu\rm m$| silicate absorption feature in narrow-line AGN are addressed. The ensuing constraints are discussed. We show that models of thick tori extending up to a few hundred parsecs in which physical processes such as shocks significantly reduce the silicate grain abundance within the first few tens of parsecs are fully consistent with available broad-band data and high-resolution IR spectra of Seyfert 1 and 2 nuclei.