SUBARCSECOND MIDINFRARED IMAGING OF WARM DUST IN THE NARROW-LINE REGION OF NGC-1068

Subarcsecond 8 and 10 mum and diffraction-limited 19 mum imaging of th e inner few hundred parsecs of the Seyfert nucleus in NGC 1068 shows t he emission to be extended over a region of approximately 70 x 140 pc. In particular, 10.3 mum images with spatial resolutions of 0.5'' or b etter reveal that the warm dust is associated with the narrow-line clo uds and is probably partially mixed with the photoionized gas. Extinct ion considerations, however, imply that the bulk of the warm dust is l ocated deeper in neighboring molecular clouds, the exposed surfaces of which form the narrow-line clouds. Since there is little evidence for ongoing massive star formation in the narrow-line region, we argue th at the mid-infrared emission arises from dust heated directly by radia tion from the central nonthermal source. No single point source is obs erved to be responsible for more than approximately 40% of the 10.3 mu m emission. The fact that the nucleus does not dominate the mid-infrar ed energy output from the nuclear environment is difficult to reconcil e with current theories which incorporate a dusty, few parsec-scale mo lecular torus as the common agent unifying the two classes of Seyfert galaxies. This difficulty is further exacerbated by the observation th at warm molecular gas is present in a region extending over 350 pc cen tered on the nucleus, and by the absence of a near-infrared excess in the spectrum which would indicate the presence of large column densiti es of hot dust close to the active galactic nucleus. As an alternative to the torus scenario, we present a model in which the molecular mate rial in the nuclear vicinity is distributed in such a way that the bul k of the gas and dust lies at relatively large distances from the nucl eus. In this case the line-of-sight extinction toward the broad-line r egion could be merely the result of one or more intervening molecular clouds. We demonstrate the plausibility of this scenario using a radia tive transfer calculation for dusty clouds which incorporates scatteri ng, an effect known to be important within approximately 100 pc of the nucleus.