On the polarization of resonantly scattered emission lines - II. Polarized emission from anisotropically expanding clouds

The linear polarization of broad emission lines from anisotropically expanding clouds in active galactic nuclei is calculated using a Monte Carlo method. Both semi-forbidden and permitted lines are considered. We investigate the effect of spatial transport and geometric anisotropy on polarization. It is found that in general higher polarization is obtained from the transitions with small angular momenta. Maximum polarizations of 8 and 4 per cent are obtained from a single stationary oblate cloud and from a stationary prolate cloud, respectively, when the optical depth $$\tau$$ is $$\sim 1$$ with large axis ratios, and this condition may be satisfied by some semi-forbidden lines that have optical depths $$\tau \sim 1-10$$ (e.g. C iii] $$\lambda$$ 1909). Similar results are obtained from expanding clouds depending on the optical depths and kinematic conditions. Numerical integration over a disc-like configuration of emission clouds shows 4 percent or lower polarization, which is lower than that of a single cloud.