Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...315l.109m&link_type=abstract
Astronomy and Astrophysics, v.315, p.L109-L112
Astronomy and Astrophysics
Astrophysics
115
Infrared: Galaxies, Galaxies: Seyfert, Galaxies: Individual: Circinus, Line: Formation
Scientific paper
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, H_2_ 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 νFnu_ 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 2x10^5^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 luminosity.
de Graauw Th.
Genzel Reinhard
Lutz Dieter
Marconi Alessandro
Moorwood Alan F. M.
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