Infrared emission in Seyfert 2 galaxies : reprocessed radiation from a dusty torus?

Abstract

The nature of the infrared continuum in Seyfert 2 galaxies is not well understood. An attractive model is one in which dust grains near the nucleus are heated by optical and ultraviolet photons from a central source and reradiate in the IR. However, a comparison of the observed optical-UV continuum with the observed IR emission indicates that in many Seyfert 2's there is a deficiency of dust heating photons. This apparent defi­ ciency may be understood if the photons responsible for heating the dust escape anisotropically. Recent work has revealed that a number of Seyfert 2 galaxies exhibit conically shaped regions of gas apparently illuminated by a collimated, nuclear ionizing source. In this Letter, we test one model for this collimation, namely that the cones result from shadowing of a compact nuclear continuum source by a thick, dusty torus. From the emission-line ratios measured for gas within the cones, we have calculated the number of ionizing photons emitted by the compact nucleus. Then, on the assumption that this compact nuclear source radiates iso­ tropically, we have found the optical-UV power incident on the torus, which is expected to be reradiated in the IR. We find the observed IRAS luminosities are consistent with the torus model in eight of the nine objects with sufficient data to perform the calculation.