Simultaneous Optical and Near‐Infrared Spectropolarimetry of Type 2 Seyfert Galaxies

Abstract

We present optical and near-infrared spectropolarimetry of the nuclei of four type 2 Seyfert galaxies, Mrk 463E, Mrk 1210, NGC 1068, and NGC 4388. The data were obtained simultaneously, covering the wavelength range of 0.46-2.5 μm. We model the polarizations from two dust-scattering components: (1) scattering in dusty regions in ionization cones and (2) scattering in a torus surrounding a type 1 nucleus. The polarizations from electron scattering in the cones and dichroic absorption by aligned dust grains in the torus are also compared with the observations. We confirmed that a combination of electron and dust scattering in the ionization cones is the preferred mechanism for the optical continuum polarization. For the near-infrared, dichroic absorption by aligned grains can explain the continuum polarization of Mrk 463E and Mrk 1210 as well as NGC 1068. Visual optical depths of the order of 10-20 are estimated for dichroic absorption in these nuclei. Dust scattering in the torus, whose grain size distribution is assumed to be the same as in the Galactic diffuse interstellar medium, cannot reproduce the observed spectral slope of the near-infrared polarization and total nuclear flux simultaneously. However, this might only indicate that the grain size distribution in the torus of active galactic nuclei (AGNs) is different, and dust scattering with moderate optical depth and dominated by large grains might provide a reasonable explanation for the near-infrared radiation from AGNs.