ISO‐SWS Spectroscopy of NGC 1068

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Abstract
We present Infrared Space Observatory-Short Wave Spectrometer (ISO-SWS) spectroscopy of NGC 1068 for the complete wavelength range 2.4-45 μm at resolving power ~1500. Selected subranges have been observed at higher sensitivity and full resolving power ~2000. We detect a total of 36 emission lines and derive upper limits for 13 additional transitions. Most of the observed transitions are fine-structure and recombination lines originating in the narrow-line region (NLR) and the inner part of the extended emission-line region. We compare the line profiles of optical lines and reddening-insensitive infrared lines to constrain the dynamical structure and extinction properties of the NLR. The most likely explanation of the considerable differences found is a combination of two effects: (1) The spatial structure of the NGC 1068 NLR is a combination of a highly ionized outflow cone and lower excitation extended emission. (2) Parts of the NLR, mainly in the receding part at velocities above systemic, are subject to extinction that is significantly suppressing optical emission from these clouds. Line asymmetries and net blueshifts remain, however, even for infrared fine-structure lines suffering very little obscuration. This may be either due to an intrinsic asymmetry of the NLR, as perhaps also suggested by the asymmetric radio continuum emission or due to a very high column density obscuring component which is hiding part of the NLR even from infrared view. We present detections and limits for 11 rotational and rovibrational emission lines of molecular hydrogen (H2). They arise in a dense molecular medium at temperatures of a few hundred kelvins that is most likely closely related to the warm and dense components seen in the near-infrared H2 rovibrational transitions, and in millimeter-wave tracers (CO, HCN) of molecular gas. Any emission of the putative parsec-scale molecular torus is likely overwhelmed by this larger scale emission. In companion papers we use the SWS data to derive the spectral energy distribution emitted by the active nucleus of NGC 1068, to put limits on infrared emission from the obscured broad-line region, and discuss the continuum and its features in conjunction with SWS spectra of other galaxies.
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