Lyα Radiative Transfer: A Stokes Vector Approach to Lyα Polarization

Abstract

Ly α- emitting galaxies and giant Ly α blobs (LABs) have been extensively observed to study the formation history of galaxies. However, the origin of their extended Ly α 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 α emission originates from the resonance scattering. The Monte Carlo Ly α radiative transfer code LaRT is updated to investigate the polarization of Ly α using the Stokes vector formalism. We apply LaRT to a few models to explore the fundamental polarization properties of Ly α . Interestingly, individual Ly α 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 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 α radiation field, which are eventually controlled by the medium’s optical depth and velocity field. Once Ly α 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 α source location. In contrast, a shallow surface brightness profile gives rise to a slowly increasing polarization pattern.