Spectroscopic constraints on the properties of dust in active galactic nuclei

The lack of a significant silicate or silicon carbide emission feature in bright active galactic nuclei (AGNs) is used to constrain models in which the IR continuum is emitted by dust. We consider two models for the dust, a graphite + silicate grain mixture and a graphite + silicon carbide mixture. The optical properties of the grains are calculated using Mie theory, the Rayleigh-Gans approximation and geometric optics, for grains in the 0.005-10 micron size range, over the 1000 micron-I A wavelength range. We use these grain models to calculate the emission of optically thin and of optically thick dust, with various grain compositions, incorporating both absorption and scattering in the detailed radiative transfer. We find that ~1:1 mixtures of graphite + silicate grains of a 0.01. (4) If the IR emission originates in clouds which are optically thick at 10 microns, then U ~> 0.1 at the cloud surface. We finally note that physical and dynamical arguments lead to similar constraints: small grains in an optically thin dust configuration are likely to be destroyed on a short time scale, while very large grains and grains in an optically thick cloud, both of which do not produce a pronounced emission feature, or reddening, are likely to survive longer.