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Astron. Astrophys. 315, L109-L112 (1996)

2.5-45 tex2html_wrap_inline40m SWS spectroscopy of the Circinus Galaxy*

A.F.M. Moorwood1, D. Lutz2, E. Oliva3, A. Marconi4, H. Netzer5, R. Genzel2, E. Sturm2, and Th. de Graauw6

1 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
2 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
3 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
4 Universitá degli Studi di Firenze, Dip. di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, I-50125 Firenze, Italy
5 School of Physics and Astronomy and the Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
6 SRON, Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 16 July 1996 / Accepted 30 August 1996

Abstract. We present 2.5-45 img1.gifm 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, H img2.gif 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 5000 cm-3 derived from the [NeV]14.3/24.3 img1.gifm 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 img4.gif 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] 26 img1.gifm and [SIV] 10.5 img1.gifm which are observed to be a factor img4.gif 2 brighter and fainter respectively than expected. The best fit ionizing spectrum is characterized by a img5.gif X-ray continuum - consistent with recent ASCA satellite observations - plus a pronounced UV bump which peaks at img6.gif 70eV (180Å) in a plot of img7.gif F img8.gif versus img7.gif 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 2 x 10 img9.gif 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 luminosity.

Key words: infrared: galaxies - galaxies: Seyfert - galaxies: individual: Circinus - line: formation

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.

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