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

We compute the polarization of the Lyman alpha line photons emerging from ananisotropically expanding and optically thick medium, which is expected tooperate in many Lyman alpha emitting objects including the primeval galaxyDLA~2233+131 and Lyman break galaxies. In the case of a highly optically thickmedium, the escape of resonance line photons is achieved by a large number ofresonant local scatterings followed by a small number of scatterings in thedamping wing. We show that some polarization can develop because the wingscatterings are coupled with strong spatial diffusion which depends on thescattering geometry and kinematics. The case of a slab with a finite scatteringoptical depth and expansion velocity of ~ 100 kms^{-1} is investigated and itis found that Lyman alpha photons are emergent with the linear degree ofpolarization up to 10% when the typical scattering optical depth tau>=10^5. Wesubsequently investigate the polarization of Lyman alpha photons emerging froma spherical shell obscured partially by an opaque component and we obtain ~ 5 %of polarization. It is proposed that a positive detection of polarized Lymanalpha with with P-Cygni type profile from cosmological objects can be a strongtest of the expanding shell structure obscured by a disk-like component.Comment: Submitted to ApJ Lette