2.5-45 MU-M SWS SPECTROSCOPY OF THE CIRCINUS GALAXY

We present 2.5-45 mu m ISO SWS spectra of the Circinus galaxy which is one of the closest spirals containing both a visually obscured Seyfer t nucleus and circumnuclear starburst activity. In addition to PAH fea tures, H-2 and low excitation ionic lines ([FeII], [NeII], [SIII]) fro m the starburst region its spectrum shows several prominent lines from highly ionized species ([NeIII,V,VI], [SIV], [MgV,VII,VIII], [OIV], [ SiIX]) which arise in gas photoionized by the AGN within its ionizatio n cone. Adopting solar abundances and a density of 5000 cm-3 derived f rom the [NeV]14.3/24.3 mu m ratio we have developed a model which can reproduce the absolute fluxes of most of the visible/near infrared and ISO lines from species with ionization energies in the range 50 - 320 eV to within a factor similar to 2. Most of the discrepancies can pro bably be attributed to uncertainties in the calibration, abundances an d atomic parameters. Of particular interest for further work are [OIV] 26 mu m and [SIV] 10.5 mu m which are observed to be a factor similar to 2 brighter and fainter respectively than expected. The best fit io nizing spectrum is characterized by a v(-0.5) X-ray continuum - consis tent with recent ASCA satellite observations - plus a pronounced UV bu mp which peaks at similar or equal to 70eV (180 Angstrom) in a plot of vF(v) versus v and is responsible for most of the ISO lines. Models i n which the UV bump is represented by a broken power law, typical of a thin accretion disc, or a quasi black body at 2 x 10(5)K give equally good fits to the observed emission line spectrum. Whatever its origin it appears that re-processed ionizing radiation from the AGN probably dominates over the starburst contribution to the total infrared lumin osity.