Polarimetry and modelling of narrow-line active galaxies

Abstract

We present optical spectropolarimetry along with optical and infrared broad-band filter polarimetry of selected warm [f(25 μm)/f(60 μm) ≥ 0.3] IRAS galaxies and other known Seyfert 2 galaxies. Broad lines in polarized flux have been detected in a number of type 2 IRAS galaxies. From a determination of the intrinsic polarization of the scattered radiation it has been possible to model the optical and near-IR flux density and degrees of polarization, for a number of objects, with a cone-based scattering geometry. In all these cases an additional polarizing mechanism was required to match the near-IR data, and this was successfully modelled by a dichroic view of the near-IR emitting regions through the postulated torus which surrounds the type 1 core. For those objects which show broad lines in polarized flux, and for which the intrinsic polarization of the scattered radiation could be calculated, the inclinations to the line of sight tend to be low. The broad Hα luminosities calculated for the IRAS galaxies are more typical of QSOs than Seyfert 1 galaxies. Only a fraction of the IRAS galaxies observed exhibit broad lines in polarized flux indicating that either the scattering region is also obscured, or the broad lines are thermally broadened and rendered unobservable, or the unified theory is incorrect. Evidence supporting the former case is presented.