Seyferts on the edge: Polar scattering and orientation-dependent polarization in Seyfert 1 nuclei

Abstract

We have identified 12 Seyfert 1 galaxies that exhibit optical polarization spectra similar to those of Seyfert 2 galaxies in which polarized broad lines are detected. We present new spectropolarimetric observations of three of them: Was 45, Mrk 231 and NGC 3227. These objects appear to be polarized as a result of far-field scattering in the polar illumination cones of the circumnuclear torus. We estimate that they represent between 10 and 30 per cent of the Seyfert 1 population; they are found amongst all the main spectroscopic subtypes, including narrow-line Seyfert 1 galaxies. We have shown elsewhere that Seyfert 1 nuclei more commonly have polarization characteristics that can be attributed to scattering by a compact 'equatorial' scattering region located inside the torus. We propose that both equatorial and polar scattering regions are present in all Seyfert galaxies and argue that the observed range of polarization properties can be broadly understood as an orientation effect. In this scheme, polar-scattered Seyfert 1 galaxies represent the transition between unobscured (the majority of type 1) and obscured (type 2) Seyferts. They are viewed through the upper layers of the torus and are thus subject to moderate extinction (Av ≈ 1-4 mag) sufficient to suppress polarized light from the equatorial scattering region, but not the broad wings of the Balmer lines. The orientation of the polarization position angle relative to the radio source is broadly consistent with the two-component scattering model. More generally, we find that amongst Seyfert 1 galaxies, parallel, perpendicular and intermediate orientations of the polarization position angle relative to the radio axis occur roughly in the proportions 2:1:1.