Astron. Astrophys. 329, L21-L24 (1998)

1. Introduction

After the discovery of polarized broad lines in NGC1068, spectropolarimetry has become a standard tool to trace hidden broad line regions (BLR) in objects like type 2 Seyferts where BLR and AGN continuum are visually obscured (cf. Antonucci 1993 for a review). In the standard model 1% of the BLR photons are scattered toward us by a `mirror' (dust or free electrons) lying outside the obscuring `torus', and the light appears partly polarized. Seyferts 2 with polarized BLR are usually characterized by prominent narrow lines onto a mixture of stellar and non stellar (featureless) continua, and the latter accounts for a significant fraction ( 10%) of the total observed continuum flux. The narrow lines are much less polarized than the continuum and [OIII] is often (but not always) unpolarized. The stellar flux is also basically unpolarized and is subtracted using suitable stellar templates to obtain the `true' polarization of the non stellar nuclear emission (e.g. Miller & Goodrich 1990).

The Circinus galaxy is an anonymous, nearby ( 4 Mpc) spiral lying very close to the galactic disk (b =-3.8 ) in a region of relatively low Galactic extinction (A 1.5 magnitudes, Freeman et al. 1977). Its Seyfert 2 activity was already suspected on the basis of IR colours (Moorwood and Glass, 1984) and definitely demonstrated by recent observations of optical and IR coronal lines (Oliva et al. 1994, hereafter O94, Moorwood et al. 1996), optical line images showing a spectacular [OIII] cone and H starburst ring (Marconi et al. 1994, hereafter M94), and X-rays spectroscopy revealing a very prominent Fe-K line (Matt et al. 1996). Being very bright and 5 times closer than NGC1068, the Circinus galaxy is the ideal laboratory to study the Seyfert 2 phenomenon. The only drawback is that the object cannot be easily studied in the UV because the central 3" regions suffer by significant local extinction (A 3 mag), and this absorption strongly varies with position. To our knowledge, no spectropolarimetric observations of this galaxy exist.

This letter presents for the first time spectropolarimetric observations of this galaxy. The observations and results are described in Sect. 2, 3 while Sect. 4 presents a simple model which is also used to estimate the intrinsic properties of the obscured BLR.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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