Subarcsecond Mid-Infrared Imaging of Warm Dust in the Narrow-Line Region of NGC 1068

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Abstract

Subarcsecond 8 and 10 microns and diffraction-limited 19 microns imaging of the inner few hundred parsecs of the Seyfert nucleus in NGC 1068 shows the emission to be extended over a region of ∼70 × 140 pc. In particular, 10.3 microns images with spatial resolutions of 0"5 or better reveal that the warm dust is associated with the narrow-line clouds and is probably partially mixed with the photoionized gas. Extinction considerations, however, imply that the bulk of the warm dust is located 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 that the mid-infrared emission arises from dust heated directly by radiation from the central nonthermal source. No single point source is observed to be responsible for more than ∼40% of the 10.3 microns emission. The fact that the nucleus does not dominate the mid-infrared energy output from the nuclear environment is difficult to reconcile with current theories which incorporate a dusty, few parsec-scale molecular torus as the common agent unifying the two classes of Seyfert galaxies. This difficulty is further exacerbated by the observation that warm molecular gas is present in a region extending over 350 pc centered on the nucleus, and by the absence of a near-infrared excess in the spectrum which would indicate the presence of large column densities of hot dust close to the active galactic nucleus. As an alternative to the torus scenario, we present a model in which the molecular material in the nuclear vicinity is distributed in such a way that the bulk of the gas and dust lies at relatively large distances from the nucleus. In this case the line-of-sight extinction toward the broad-line region could be merely the result of one or more intervening molecular clouds. We demonstrate the plausibility of this scenario using a radiative transfer calculation for dusty clouds which incorporates scattering, an effect known to be important within ∼100 pc of the nucleus.

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