| Most active galactic nuclei are characterized by prominent UV emission lines in their spectra, and the line photons are believed to be resonantly scattered many times before emerging along the line of sight. The polarization of these line photons, which is sensitive to the atomic physics, can be an important tool for investigating the scattering geometry and kinematics of the line-emitting region. The principle of resonance scatterings are discussed in order to elucidate the relation between the emergent polarization, the atomic transitions and the anisotropy of the radiation field. When the ratio of collision frequency to radiative excitation rate is much larger than 1, collisional mixing occurs and the ground state sublevels are equally populated. The opposite case is radiative mixing, where the anisotropic radiation field may induce uneven population in the ground state sub-levels, leading to enhancement of polarization. A density matrix is introduced to deal with the photon polarization state and the level population of the ions.; A density matrix formalism based Monte Carlo approach is adopted and used to compute the polarization of emission lines emerging from anisotropic clouds or anisotropically expanding clouds. The effect of both spatial diffusion and geometric anisotropy on polarization is considered. It is shown that semi-forbidden C III) {dollar}lambda{dollar}1909, which has a moderate optical depth ranging 1-10 in the broad emission line region, can be polarized, whereas most permitted UV lines having large scattering optical depth are negligibly polarized.; The same code is modified to incorporate the Sobolev approximation and polarization of the broad absorption line troughs in broad absorption line quasars. Both bipolar flow and equatorial flow are examined. Up to 15 percent polarization in the absorption trough is obtained in the doublet transition J = 1/2 {dollar}to{dollar} 1/2, 3/2 for an equatorial flow model, and from a bipolar flow model, a lower degree of polarization is obtained. A higher polarization is obtained for the singlet transition case J = 0 {dollar}to{dollar} 1, which should be a good diagnostic to test whether the polarization is caused by resonance scattering. Recent spectropolarimetry is briefly summarized, and observational ramifications are discussed.; Finally the conditions necessary to produce significant polarization from out-flowing gas from quasars are studied. Multiplet transitions with radiative mixing are considered and compared to the polarization from a singlet transitions which produce the highest polarization. The steep rise in the polarized flux shortward of {dollar}sim{dollar}800A observed in a radio quiet quasar PG 1630 + 377 is interpreted in terms of resonance line scattering. |
