Simultaneous Optical and Near-Infrared Spectropolarimetry of Type 2 Seyfert Galaxies with TRISPEC

Abstract

We present 0.46{2.5 ¹m spectropolarimetry of four type 2 Seyfert nuclei, Mrk 463E, Mrk 1210, NGC 1068, and NGC 4388, obtained with TRISPEC (Triple Range Imager and SPECtrograph) at UKIRT. The polarizations from dust or electron scattering in ionization cones, dust scattering in a torus surrounding a type 1 nucleus, and dichroic absorption by aligned dust grains in the torus are modeled. In the near-infrared, dichroic absorption (AV = 10{ 20) by aligned grains can explain the continuum polarization of Mrk 463E and Mrk 1210, as well as that of NGC 1068. Dust scattering in the torus, whose grain size distribution is assumed to be the same as in the Galactic di®use interstellar medium, cannot reproduce the observed spectral slope of the near-infrared polarization and total nuclear °ux simultaneously. However, this might only indicate that the grain size distribution in the torus of AGNs is di®erent, and dust scattering with moderate optical depth and dominated by large grains might provide a reasonable explanation for the near-infrared radiation from AGNs.