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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
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 (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
Send offprint requests to: E. Oliva
Online publication: December 22, 1998