Unification Model and Rayleigh Scattered Lyα in Active Galactic Nuclei

The unification model of active galactic nuclei invokes the presence of a thick molecular torus that hides the broad emission line region from a line of sight toward observers with low latitude. It is expected that the illuminated side of the molecular torus may be photodissociated by strong far UV radiation from the central AGN, forming an H I region with a high neutral column density. We propose that the Rayleigh scattering optical depth of this HI region can be significant for most broad $Ly{\alpha}$ line photons with the Doppler factor not exceeding 104 km s-1. Rayleigh scattered $Ly{\alpha}$ photons can be characterized by strong linear polarization depending on their scattering optical depth. We performed Monte Carlo simulations of polarized radiative transfer of $Ly{\alpha}$ adopting simple scattering geometries relevant to the unification model of AGN. We find that for a low torus the Rayleigh scattered $Ly{\alpha}$ is polarized in the direction parallel to the symmetry axis with the polarization degree dependent on wavelength. In the case of a high torus, the core part of $Ly{\alpha}$ is polarized in the direction perpendicular to the symmetry axis whereas the wing part is parallelly polarized. We conclude that careful spectropolarimetry around $Ly{\alpha}$ can be useful in testing the AGN unification model.