Spatially Resolved Mid‐Infrared Spectroscopy of NGC 1068: The Nature and Distribution of the Nuclear Material

We present spatially-resolved, near-diffraction-limited 10 micron spectra ofthe nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, themid-IR imager and spectrometer on the 8.1 m Gemini North telescope. The spectracover the nucleus and the central 6.0" x 0.4" of the ionization cones at aspatial resolution of approximately 0.4" (approx. 30 parsecs). The spectraextracted in 0.4" steps along the slit reveal striking variations in continuumslope, silicate feature profile and depth, and fine structure line fluxes onsubarcsecond scales, illustrating in unprecedented detail the complexity of thecircumnuclear regions of this galaxy at mid-IR wavelengths. A comparison ofphotometry in various apertures reveals two distinct components: a compact(radius <15 pc), bright source within the central 0.4" x 0.4" and extended,lower brightness emission. We identify the compact source with the AGNobscuring torus, and the diffuse component with the AGN-heated dust in theionization cones. While the torus emission dominates the flux observed in thenear-IR, the mid-IR flux measured with apertures larger than about 1" isdominated instead by the dust emission from the ionization cones. Many previousattempts to determine the torus spectral energy distribution are thus likely tobe significantly affected by contamination from the extended emission. Theobserved spectrum of the compact source is compared with clumpy torus models,which require most of the mid-IR emitting clouds to be located within a fewparsecs of the central engine. We also present a UKIRT/CGS4 5 micron spectrumcovering the R(0) -- R(4) lines of the fundamental vibration-rotation band of12CO. None of these lines was detected, and we discuss these non-detections interms of the filling factor and composition of the nuclear clouds. (Abridged)Comment: 35 pages, 8 figures; scheduled for publication in 2006, ApJ, 640,