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

4. Discussion

4.1. Origin of the continuum polarization

The observed 2% continuum polarization is very difficult to interpret in terms of scattered nuclear light diluted by unpolarized stellar emission because this would require an unreasonably large ( 70%) intrinsic polarization. Besides, in such a scenario the P spectra should contain narrow [OIII], [SII] absorption, unless the NLR intrinsic polarization is finely tuned to avoid this effect.

The most natural interpretation is that all the light (i.e. stars, NLR and scattered BLR) is weakly polarized by either transmission through, or single scattering by the dust in the galactic disk. A highly simplified though plausible scenario is that sketched in Fig. 4 where the observer sees the extincted radiation from the stars, the NLR and the BLR mirror, plus a small fraction of `off-axis' light scattered toward the line of sight by the dust in the disk of the Circinus galaxy. A non-zero polarization is ensured by the asymmetry of the dust distribution whose column density increases toward SE producing the observed sharp extinction gradient, and by the non uniform distribution of the stellar and NLR emission which is strongly peaked on the nucleus (cf. M94). The shape of the observed P spectrum depends on the relative contribution of scattering and transmission polarizations, and the latter is more important in the highly extincted SE region thus producing a flatter P spectrum.

Fig. 4. Left: sketch of the model used to interpret the observed polarization of the stellar and NLR spectra. The observer mostly sees direct (extincted) radiation plus a small fraction of off-axis light scattered toward the line of sight by dust in the disk. Right: results of the model discussed in Sect. 4.2, the complete spectra are at the EFOSC1 resolution while the details around H are the predicted spectra at resolving power R =2000.

The fact that the corrected polarization angle is basically constant with and does not show significant variations within the broad H profile requires that the polarization by the BLR mirror and scattering/transmitting dust have similar position angles. This is not unreasonable because the corrected is 20 , roughly parallel to the disk and dust lane and perpendicular to the axis of the [OIII] cone (cf. Figs. 1, 2 of M94). If the BLR mirror is along the cone axis and the magnetic field is aligned with the galaxy disk, it naturally follows that all polarization angles should be similar.

4.2. The physical properties of the BLR and its mirror

A detailed treatment of the radiation transfer and scattering through the disk is beyond the aims of this paper. Here we concentrate on the properties of the BLR scattered spectrum and make the reasonable assumption that the polarization introduced by transmission/scattering through the disk could be parameterized by where the slope is 2 when single scattering by small grains dominates, while is 0 when the main polarization mechanism is transmission through aligned non-spherical grains. If the polarization angles are similar, and as long as the polarization degrees are small (which is our case), the observed degree of linear polarization is simply the sum of and , the polarization observed by ideally removing the galaxy disk. We model assuming that the nuclear scattered spectrum is polarized, while both the stellar continuum and narrow lines are unpolarized.

The results of a toy model with a 1% scattered AGN are shown in right hand panel of Fig. 4. Due to the large dilution, the value of is very small at all wavelengths but at the positions of the broad H whose amplitude is a factor 5 the nuclear continuum level and therefore stands out in because it is 5 times less diluted than the surrounding continuum, and the same applies to the weaker H . The narrow lines appear in absorption because they further dilute the scattered spectra, but their amplitude is very small simply because the the continuum is very low. The effect of the disk polarization is simply to add a smooth continuum to , and does not affect the amplitude of the emission (BLR) and absorption (NLR) lines. Details of the model of Fig. 4 are as follows.
- Nuclear continuum: scattered by a gray mirror
- W (H -broad)=400 Å ; I(H - broad)/I(H -broad) = 3.5
- BLR mirror extincted by A =5 mag (same as NLR, cf. O94)
- Scattered spectrum has P =25% at all wavelengths
- Nuclear scattered continuum is 1% of observed
- Stellar and NLR spectra are from high resolution EMMI data
- Polarization by the disk:
These values are by no means unique because similarly good fits could be obtained by e.g. decreasing the intrinsic equivalent width of H and increasing the contribution by the BLR to the total observed continuum, but the fit rapidly deteriorates for equivalent widths below 200 Å and W  Å could be considered a tight lower limit.

A quite firm result is that the scattered light cannot be much bluer than assumed otherwise the broad H would appear too strong. This sets a tight lower limit A 2 for the extinction suffered by the BLR mirror, but gives no useful information on the intrinsic spectral efficiency of the mirror whose extinction is unknown. In other words, the scattered light could equally well come from a `gray mirror' suffering an extinction similar to the NLR (the assumption of Fig. 4) or from a `blue mirror' suffering a larger exctinction.

The best constrained parameter is the observed flux of broad H which is erg cm^-2 s^-1 and translates into if the extinction toward the BLR mirror is similar to that measured for the narrow lines. Assuming a `standard' mirror efficiency of 1%, the intrinsic luminosity of broad H becomes or 0.5% of the IRAS FIR luminosity, a ratio similar to those found in many type 1 Seyferts (see e.g. Table 1 of Ward et al. 1988).

Interesting predictions of the model are the detection of circular polarization due to scattering of the linearly polarized BLR, and high resolution P spectra which should reveal a stronger H broad component with sharp absorption features at the positions of the narrow [NII] and H lines (Fig. 4)

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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