Astron. Astrophys. 325, 450-456 (1997)

1. Introduction

The Circinus galaxy (A1409-65) is a large ( 17') spiral of type S(b-d) at a distance 4 Mpc which is located in a region of low extinction close to the galactic plane (Freeman et al., 1977). It is of particular interest as the closest example of a galaxy containing an obscured Seyfert nucleus surrounded by circumnuclear starburst activity. Evidence for the Seyfert activity is provided by its spectacular ionization cone (Marconi et al., 1994); prominent high excitation coronal line spectrum (Oliva et al., 1994; Moorwood et al., 1996, hereafter M96) and hard X-ray continuum (Matt et al., 1996). Star formation activity traced by H (Marconi et al., 1994) and Br recombination line emission (Moorwood & Oliva, 1994) peaks in a partial ring of radius 200pc. Based on the nuclear continuum deduced from its emission line spectrum and the ionization rate inferred for the starburst ring, Moorwood et al. (1996) have suggested that the starburst may contribute only 10% of the total luminosity rather than the 70% inferred previously by Rowan-Robinson & Crawford (1989) from a decomposition of its spectral energy distribution in the IRAS bands. Nevertheless, pronounced PAH features, normally associated with the presence of hot stars, are seen from the ground at 3.28µm within a 7.5" aperture (Moorwood, 1986) and at 6.2, 7.7, 8.8 and 11.3µm within a 14 x 20" aperture with ISO (M96). Both the ground-based and ISO spectra also show pronounced 9.7µm silicate absorption. The overall infrared 1.2 - 100µm spectral energy distribution based on ground-based and IRAS observations has been published by Moorwood & Glass (1984) and the recently obtained 2.5 - 45µm spectrum obtained with ISO appears in M96.

In this paper we present measurements of the size of the infrared and millimeter emitting region and test, using the radiative transfer code developed by Krügel & Siebenmorgen (1994, hereafter KS94), if the size versus wavelength; overall spectral energy distribution and presence of PAH features can be modelled assuming that the dust heating is dominated by a central power law source.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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