1996A&A...315L.109M

We present 2.5-45µm ISO SWS spectra of the Circinus galaxy which is one of the closest spirals containing both a visually obscured Seyfert nucleus and circumnuclear starburst activity. In addition to PAH features, H₂ and low excitation ionic lines ([FeII], [NeII], [SIII]) from 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 ionization cone. Adopting solar abundances and a density of 5000cm^–3 derived from the [NeV]14.3/24.3µm 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 50-320eV to within a factor ∼2. Most of the discrepancies can probably be attributed to uncertainties in the calibration, abundances and atomic parameters. Of particular interest for further work are [OIV] 26µm and [SIV] 10.5µm which are observed to be a factor ∼2 brighter and fainter respectively than expected. The best fit ionizing spectrum is characterized by a ν^–0.5 X-ray continuum - consistent with recent ASCA satellite observations - plus a pronounced UV bump which peaks at ≃70eV (180Å) in a plot of νF_ν versus ν and is responsible for most of the ISO lines. Models in which the UV bump is represented by a broken power law, typical of a thin accretion disc, or a quasi black body at 2x10⁵K 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 luminosity.