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Astron. Astrophys. 342, 87-100 (1999)

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Metal abundances and excitation of extranuclear clouds in the Circinus galaxy *

A new method for deriving abundances of AGN narrow line clouds

E. Oliva 1, A. Marconi 1 and A.F.M Moorwood 2

1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany

Received 23 July 1998 / Accepted 4 November 1998


Spectra of extranuclear clouds within the ionization cone of Circinus galaxy are presented and modelled assuming photoionization by the Seyfert 2 nucleus but no preconceived assumption concerning the spectral shape of the ionizing AGN radiation or the gas density distribution.

The most important result is that, regardless of the assumed AGN spectral shape or density distribution, the metal abundances are remarkably well ([FORMULA]0.2 dex) constrained by the data. Oxygen and neon are found to be a factor of 5 below solar, while nitrogen is solar (cf. Table 6). The large N/O overabundance is in agreement with the chemical enrichment expected from the old (circum)nuclear starburst. Similar studies of extranuclear clouds in other objects could therefore provide a powerful tool to determine metallicities and trace past starburst activity in AGNs.

Other implications such as the role of shock excitation, which is effectively excluded, and the intrinsic shape of the AGN ionizing continuum, which is poorly constrained by the data, are also discussed.

Key words: galaxies: abundances – galaxies: active – galaxies: individual: Circinus – galaxies: starbust

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: E. Oliva

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998