Interstellar polarization and the position-angle orientations of type 1 Seyfert galaxies

Abstract

We comment on recent spectropolarimetric studies that compare the observed polarization position angles (PAs) of type 1 Seyfert galaxies near Hα with the observed orientations of their radio-source axes on the sky. For a Seyfert galaxy in which scattering occurs mainly in an equatorial scattering region, the polarization PA is expected to be parallel to the radio axis, while for a case in which light scatters predominantly in the polar regions, the Hα polarization PA should be perpendicular to the radio axis. In practice, these correlations are difficult to establish because a Galactic interstellar polarization contribution can introduce a significant uncertainty into the polarization PA determination, even when the magnitude of interstellar polarization is small. We show how such uncertainties may affect the analysis of PA alignments, and present spectropolarimetric observations of a probe star along the line of sight to the type 1 Seyfert galaxy Mrk 871 that allow us to assess the intrinsic Hα polarization and PA of Mrk 871. These results suggest that spectropolarimetric observations of such probe stars should form an integral part of future polarization studies of Seyfert galaxies.