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

Open Access
Publisher Website
Google Scholar
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.