Astronomy and Astrophysics, volume 315, L109-112 (1996/11-2)
2.5-45µm SWS spectroscopy of the Circinus Galaxy.
MOORWOOD A.F.M., LUTZ D., OLIVA E., MARCONI A., NETZER H., GENZEL R., STURM E. and DE GRAAUW T.
Abstract (from CDS):
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 νFν 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.
infrared: galaxies - galaxies: Seyfert - galaxies: individual: Circinus - line: formation
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