Spatially-resolved mid-infrared spectroscopy of NGC 1068: the nature and distribution of the nuclear material

Abstract

We present spatially-resolved, near-diffraction-limited 10 micron spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini North telescope. The spectra cover the nucleus and the central 6.0" x 0.4" of the ionization cones at a spatial resolution of approximately 0.4" (approx. 30 parsecs). The spectra extracted in 0.4" steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of this galaxy at mid-IR wavelengths. A comparison of photometry 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 AGN obscuring torus, and the diffuse component with the AGN-heated dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1" is dominated instead by the dust emission from the ionization cones. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed 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 few parsecs of the central engine. We also present a UKIRT/CGS4 5 micron spectrum covering the R(0) -- R(4) lines of the fundamental vibration-rotation band of 12CO. None of these lines was detected, and we discuss these non-detections in terms of the filling factor and composition of the nuclear clouds. (Abridged)