A first 2.4-45 µm spectrum of the prototypical Seyfert 2 galaxy NGC 1068 has been obtained with the Short Wavelength Spectrometer SWS on board the Infrared Space Observatory ISO (see figure). The measured fine-structure line fluxes show a very similar trend with ionization potential as those measured for the Circinus galaxy (Moorwood et al. 1996) and hence can probably be fit successfully by a simple photoionization model invoking an EUV bump in the ionizing continuum. Differences are observed between the [O IV] 26 µm NLR line profile and optical NLR line profiles which may indicate significant extinction to part of the NLR or variations in electron temperature.
We detect pure rotational transitions of molecular hydrogen that must be emitted by molecular gas spanning a wide range of temperatures. The unusual strength of the fundamental S(0) 28 µm rotational transition is evidence for a large ( > 1.5×109Msol) gas mass at temperatures near 100 K. Either most of the gas in the circumnuclear region of NGC 1068 is warm or previous molecular mass estimates based on CO observations were too low.
Strong mid-infrared continuum from the circumnuclear warm dust is prominent in our spectrum. PAH emission is weak, supporting the idea that the carriers of this feature are largely destroyed by the intense AGN radiation field. The fact that we still detect these features at the edges of the 9.7 µm silicate absorption should be considered in interpretations of the silicate feature.
Moorwood, A.F.M., Lutz, D., Oliva, E., et al. 1996, A&A, 315, L109