By modelling the rich spectrum of an extranuclear cloud in the ionization cone of the Circinus galaxy we have found that metal abundances are remarkably well constrained, regardless of the assumptions made on the shape of the ionizing continuum and gas distribution. This new result may open a new and interesting field of research using photoionization models to derive metallicities in AGNs which could in turn be related to the star formation activity in the recent past, i.e. old nuclear starbursts. In the case of Circinus, the large N/O overabundance found here is fully compatible with what expected from chemical evolution models of starbursts (Sect. 5.3).
Much less encouraging are the results on the AGN ionizing continuum whose shape cannot be constrained by the observed line ratios but depends on the assumed gas density distribution. Within the limits of the model parameters spanned here we somewhat favour an AGN spectrum with a "UV-bump" but cannot exclude that, with a different and better tuned combination of density and radiation bounded clouds, one could achieve similarly good fits with a power law AGN continuum (Sect. 4.3).
We also found that photoionization models cannot reproduce the observed [FeVII]/[FeII] and [NII]/[NI] ratios and argued that these may reflect errors in the collision strengths for [FeVII] and rate coefficient of N-H charge exchange reactions. It should be noted, however, that the problem [NII]/[NI] does not significantly influence the derived nitrogen abundance.
Finally, our data strongly indicate that shocks cannot play any important role in exciting the gas producing the observed line emission.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998