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

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1. Introduction

The Circinus galaxy (A1409-65) is a nearby ([FORMULA]4 Mpc) gas rich spiral lying close to the galactic plane in a region of relatively low ([FORMULA]1.5 mag) interstellar extinction (Freeman et al. 1977). Several observed characteristics indicate that this galaxy hosts the nearest Seyfert 2 nucleus known. These include optical images showing a spectacular [OIII] cone (Marconi et al. 1994 hereafter M94); optical/IR spectra rich in prominent and narrow coronal lines (Oliva et al. 1994, hereafter O94, Moorwood et al. 1996, hereafter M96); X-ray spectra displaying a very prominent Fe-K fluorescent line (Matt et al. 1996) and optical spectropolarimetric data which reveal relatively broad H[FORMULA] emission in polarized light (Oliva et al. 1998). Complementary to these is observational evidence that this galaxy has recently experienced a powerful nuclear starburst which is now traced by the near IR emission of red supergiants (Oliva et al. 1995, Maiolino et al. 1998) and which may have propagated outwards igniting the bright ring of O stars and HII regions visible in the H[FORMULA] image (M94). Such a starburst could have been triggered by gas moving toward the nuclear region and eventually falling onto the accretion disk around the black hole powering the AGN.

A debated issue is whether nuclear starbursts are common features of AGNs and if they are more common in type 2 than in type 1 Seyferts, as suggested by e.g. 10µm observations (Maiolino et al. 1995) and studies of the stellar mass to light ratios (O94). Since starbursts are predicted and observed to deeply modify the chemical abundances of the host galaxy (e.g. Matteucci & Padovani 1993), such an effect should also be evident in this and other Seyferts. However, to the best of our knowledge, no reliable measurement of metallicity for the narrow line region clouds of Seyfert 2's exists in the literature. In particular, although it has been known for long that the large [NII]/H[FORMULA] ratio typical of Seyferts cannot be easily explained using simple models with normal nitrogen abundances (e.g. Osterbrock 1989, Komossa & Schulz 1997), the question of whether its absolute (N/H) or relative (e.g. N/O) abundance is truly different than solar is still open. Finding a reliable method to derive metallicities and, therefore, to trace and put constraints on past starburst activity is the main aim of this paper.

We chose the Circinus galaxy as a benchmark because its emission line spectrum is characterized by remarkably narrow ([FORMULA]150 km s-1, O94) emission lines which are particularly easy to measure and which indicate relatively low dynamical activity. This last aspect may be used to put tight constrains on the possible contribution of shock excitation which may complicate the modelling of the observed spectrum and the determination of metallicities.

This paper presents new optical and infrared spectroscopic data and is structured as follows. Observations and data reduction are described in Sect. 2 and the results are analyzed in Sect. 3. In Sect. 4 we constrain the excitation conditions of the gas and model the observed spectra in terms of photoionization from the AGN. The derived chemical abundances are discussed in Sect. 5 and in Sect. 6 we draw our conclusions.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998