We report on the analysis of the first complete far-infrared spectrum ( 43-197 \mu m ) of the Seyfert 2 galaxy NGCÂ 1068 as observed with the Long Wavelength Spectrometer ( LWS ) onboard the Infrared Space Observatory ( ISO ) .
In addition to the 7 expected ionic fine structure emission lines , the OH rotational lines at 79 , 119 and 163 \mu m were all detected in emission , which is unique among galaxies with full LWS spectra , where the 119 \mu m line , when detected , is always in absorption .
The observed line intensities were modelled together with ISO Short Wavelength Spectrometer ( SWS ) and optical and ultraviolet line intensities from the literature , considering two independent emission components : the AGN component and the starburst component in the circumnuclear ring of \sim 3 kpc in size .
Using the UV to mid-IR emission line spectrum to constrain the nuclear ionizing continuum , we have confirmed previous results : a canonical power-law ionizing spectrum is a poorer fit than one with a deep absorption trough , while the presence of a big blue bump is ruled out .
Based on the instantaneous starburst age of 5 Myr constrained by the Br \gamma equivalent width in the starburst ring , and starburst synthesis models of the mid- and far-infrared fine-structure line emission , a low ionization parameter ( U=10 ^ { -3.5 } ) and low densities ( n=100 cm ^ { -3 } ) are derived .
Combining the AGN and starburst components , we succeeded in modeling the overall UV to far-IR atomic spectrum of NGCÂ 1068 , reproducing the line fluxes to within a factor 2.0 on average with a standard deviation of 1.3 , and the overall continuum as the sum of the contribution of the thermal dust emission in the ionized and neutral components .
The OH 119 \mu m emission indicates that the line is collisionally excited , and arises in a warm and dense region .
The OH emission has been modeled using spherically symmetric , non-local , non-LTE radiative transfer models .
The models indicate that the bulk of the emission arises from the nuclear region , although some extended contribution from the starburst is not ruled out .
The OH abundance in the nuclear region is expected to be \sim 10 ^ { -5 } , characteristic of X-ray dominated regions .