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 > 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
non-monotonically increasing pattern in some cases, besides 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. If once Ly-alpha photon packets achieve ~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.
