Several authors have noted a high degree of polarization in the optical/IR spectrum of MCG-6-30-15 (e.g. Thompson & Martin 1988; Brindle et al. 1990). Polarization fractions of 4-6 per cent have been observed, with a slight tendency towards increasing polarization with decreasing wavelength. This is a significantly larger polarization than is typically found in Seyfert nuclei. The polarization is found to be aligned with the major axis of the host galaxy.
There are two possible causes for this polarization (see Kartje 1995 for a recent review of polarization mechanisms in AGN). First, a large scale alignment of dust grains along the line of sight to the central source can produce polarization via dichroic extinction (i.e. one polarization is preferentially scattered out of the line of sight by the dust grains). Secondly, scattering of flux into the observers line of sight by dust or free electrons will naturally produce polarization. In MCG-6-30-15 we have independent evidence for both scattering and the presence of line-of-sight dust. Thus, both of these mechanisms may be relevant - careful spectropolarimetry and detailed modeling will be needed to disentangle these effects.
We note that IRAS 13349+2438, the other good dusty-warm absorber candidate, also displays a very high degree of polarization (about 8 per cent). Wills et al. (1992) have found that the spectrum of the polarized light is typical of an unreddened AGN, thereby suggesting that the polarization is primarily due to scattering of the flux along a relatively dust-free path into the observers line-of-sight.