Published online by Cambridge University Press: 04 August 2010
We use the lack of a significant silicate or silicon carbide emission feature in bright AGNs at λ ∼ 10 µm to constrain the properties of the dust. We first calculate the optical properties of grains in the 0.005 – 10 µm size range, over the 1000 µm – 1Å wavelength range. We use these grain models to calculate the emission of optically thin and of optically thick dust, incorporating both absorption and scattering in the radiative transfer. A galactic dust composition in any configuration which is optically thin at 10 µm produces a very strong silicate emission feature and is clearly ruled out. We list what grain compositions, grain size distributions, and dust optical depths are consistent with the absence of a 10 µm feature. Independent dynamical arguments lead to a very similar set of constraints on the dust properties. We finally list the implications of these constraints on dust reddening, and line emission from AGNs.
About a third of the bolometric luminosity of quasars and bright Seyfert 1 galaxies, is emitted in the 1 – 100 µm range (Sanders et al. 1989). The emission mechanism is not yet established, but an attractive possibility is thermal reprocessing of the optical to far UV emission from the central continuum source by nearby dust. This hypothesis is supported by the IR spectral shape, its low amplitude variability, lack of significant polarization, and the near IR response to UV variations (e.g. Clavel, Wamsteker & Glass 1989).