Ly alpha Radiative Transfer: A Stokes Vector Approach to Ly alpha Polarization

Ly alpha-emitting galaxies and giant Ly alpha blobs (LABs) have been extensively observed to study the formation history of galaxies. However, the origin of their extended Ly alpha emission, especially of LABs, remains controversial. Polarization signals from some LABs have been discovered, and this is commonly interpreted as strong evidence supporting that the extended Ly alpha emission originates from the resonance scattering. The Monte Carlo Ly alpha radiative transfer code LaRT is updated to investigate the polarization of Ly alpha using the Stokes vector formalism. We apply LaRT to a few models to explore the fundamental polarization properties of Ly alpha. Interestingly, individual Ly alpha photon packets are found to be almost completely polarized by a sufficient number of scatterings (N (scatt) greater than or similar to 10(4)-10(5) in a static medium) or Doppler shifts induced by gas motion, even starting from unpolarized light. It is also found that the polarization pattern can exhibit a nonmonotonically increasing pattern in some cases, other than the commonly known trend that the polarization monotonically increases with radius. The polarization properties are primarily determined by the degree of polarization of individual photon packets and the anisotropy of the Ly alpha radiation field, which are eventually controlled by the medium's optical depth and velocity field. Once Ly alpha photon packets achieve similar to 100% polarization, the radial profile of polarization appears to correlate with the surface brightness profile. A steep surface brightness profile tends to yield a rapid increase of the linear polarization near the Ly alpha source location. In contrast, a shallow surface brightness profile gives rise to a slowly increasing polarization pattern.