A model for the infrared continuum spectrum of NGC 1068

Abstract

We present a model for the nuclear infrared (IR) continuum spectrum of the Seyfert galaxy NGC 1068. The torus emission is modelled in terms of the tapered disc models of Efstathiou & Rowan-Robinson, which give a good fit to the global infrared properties of active galactic nuclei. The models include the effects of a distribution of grain species and sizes and multiple scattering from dust. Our analysis is constrained by the inclination of the torus predicted by optical spectropolarimetry. We assume in particular that our line of sight is inclined to the axis of symmetry by about 35 deg, and that the half-opening angle of the cone is 30 deg. We find that the torus emission alone cannot account for the whole of the IR continuum spectrum. While this is in agreement with recent mid-IR imaging observations, which show that up to 60 per cent of the flux is not originating from the torus, our model suggests that the difference between the observed and predicted torus emission is actually much greater at near-IR wavelengths. We attribute this excess IR emission to a component of optically thin dust (A_V=0.1-0.5 mag) located in the ionization cone between the BLR and the NLR. This dust must be distributed as r^-2 in order to produce the required spectrum. Flatter density distributions peak at longer wavelengths and also produce a strong emission feature at 10mum, contrary to observations. Even with an r^-2 distribution, the grain mixture in the cone needs to be modified in order to suppress further the silicate emission feature. We suggest that this may be due to either destruction of silicate grains by shocks or the clumping of NLR dust. In addition, our model requires that the flux radiated by the central source towards the cone is at least a factor of 6 higher than that directed towards the bulk of the torus, which is naturally explained if the central source is an accretion disc. This conclusion depends mainly on the assumed inclination and opening angle of the torus, but is rather insensitive to other geometrical parameters.