Polarization of the L[CLC]y[/CLC]α Line from an Anisotropically Expanding H [CSC]i[/CSC] Shell in Primeval Galaxies

Abstract

We compute the polarization of the Lyα photons emerging from an anisotropically expanding and optically thick medium, which is expected to operate in many Lyα-emitting primeval objects, including damped Lyα absorbers and Lyman break galaxies. In the case of a highly optically thick medium, the escape of resonance-line photons is achieved by a large number of local resonant scatterings followed by a small number of damping-wing scatterings. We show that some polarization can develop because wing scatterings are coupled with strong spatial diffusion that depends on the scattering geometry and kinematics. The case of a slab with a finite scattering optical depth and an expansion velocity of ~100 km s-1 is investigated. When the typical scattering optical depth τ104, it is found that the emergent Lyα photons are linearly polarized with a degree of polarization up to 0.1 in the direction perpendicular to the slab plane. We subsequently investigate the polarization of the Lyα photons emerging from a hemispherical shell obscured partially by an opaque component, and we obtain ~5% of the polarization. We propose that the polarimetry of Lyα emission may provide important clues to the geometrical and kinematical structures of the H I shells in primeval galaxies and nearby starburst galaxies.