Non-standard Extinction by Circumnuclear Dust in Active Galactic Nuclei

Abstract

Observational data imply that Active Galactic Nuclei are likely to have a substantial amount of dust within some 0.1-100pc. This dust modifies the observed spectrum of the nucleus by reprocessing a significant fraction of its intrinsic bolometric luminosity. The details of the emerging spectrum in the infrared and in the ultraviolet are dependent on the composition of such dust. It has been argued that the comparison of such models with the IR data indicates strong depletion of silicates (Czerny, Loska and Szczerba 1991, Laor and Draine 1993). In this paper we discuss several theoretical extinction curves. We show that the empirical extinction curve for the Small Magellanic Cloud corresponds well to extinction by dust dominated by amorphous carbon grains. We study the extinction by dust in X-ray band and suggest that in some sources this extinction can be mistaken for an absorption by neutral gas with cosmic abundances and an accompanying soft X-ray excess below 0.4 keV. Finally, we show that an application of a pure carbon dust model to the well studied Seyfert galaxy NGC 6814 reveals a particularly simple form of the underlying starlight-subtracted continuum, where the spectrum (in F_nu) is slightly rising with frequency. Although our determination of the intrinsic spectrum is not necessarily unique, it clearly shows the importance of further studies of the circumnuclear dust.