Far-Infrared Emission Line and Continuum Spectrum of the Seyfert Galaxy NGC 1068

摘要

We report on the analysis of the first complete far-infrared spectrum (43 197 m) of the Seyfert 2 galaxy NGC 1068 as observed with the > Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). In addition to the seven expected ionic fine-structure emission lines, the OH rotational lines at 79, 119, and 163 m were all detected in emission, which is unique among galaxies with full LWS spectra, where the 119 m line, when detected, is always in absorption. The observed line intensities were modeled together with ISO > Short Wavelength Spectrometer (SWS) and optical and ultraviolet line intensities from the literature, considering two independent emission components: the active galactic nucleus (AGN) component and the starburst component in the circumnuclear ring of 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 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 NGC 1068, reproducing the line fluxes to within a factor of 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 m emission indicates that the line is collisionally excited and arises in a warm and dense region.