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Ly-alpha emitting galaxies and giant Ly-alpha blobs (LABs) have beenextensively observed to study the formation history of galaxies. However, theorigin of their extended Ly-alpha emission, especially of LABs, remainscontroversial. Polarization signals from some LABs have been discovered, andthis is commonly interpreted as strong evidence supporting that the extendedLy-alpha emission originates from the resonance scattering. The Monte CarloLy-alpha radiative transfer code LaRT is updated to investigate thepolarization of Ly-alpha using the Stokes vector formalism. We apply LaRT to afew models to explore the fundamental polarization properties of Ly-alpha.Interestingly, individual Ly-alpha photon packets are found to be almostcompletely polarized by a sufficient number of scatterings (N_scatt > 10^4-10^5in a static medium) or Doppler shifts induced by gas motion, even starting fromunpolarized light. It is also found that the polarization pattern can exhibit anon-monotonically increasing pattern in some cases, besides the commonly-knowntrend that the polarization monotonically increases with radius. Thepolarization properties are primarily determined by the degree of polarizationof individual photon packets and the anisotropy of the Ly-alpha radiationfield, which are eventually controlled by the medium's optical depth andvelocity field. If once Ly-alpha photon packets achieve ~100% polarization, theradial profile of polarization appears to correlate with the surface brightnessprofile. A steep surface brightness profile tends to yield a rapid increase ofthe linear polarization near the Ly-alpha source location. In contrast, ashallow surface brightness profile gives rise to a slowly increasingpolarization pattern.

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