On the polarization of resonantly scattered emission lines -- III. Polarization of quasar broad emission lines and broad absorption line troughs

Abstract

The contribution to the expected linear polarization of quasar broad emission and absorption lines from resonance scattering is computed using a Monte Carlo approach for specific, generic models. Attention is focused on the external illumination of the scattering region. The polarization of the reflected component from an externally illuminated slab with finite optical depth is first considered as a model of a single, dense, broad emission-line cloud. A polarization ≾ 40 per cent (≾10 per cent) is typically computed for J=0→1 (J=1/2ₒ3/2) transitions. Integration over a population of emission-line clouds typically reduces the observed polarization by a factor ∼(0.1–0.3)q_(BELR), where q_(BELR) is the covering factor for emission lines, while preserving the relative polarizations of different lines. Consequently, the low degree of polarization observed in the red wings of quasar emission lines limits the density of low column density emission-line clouds. Broad absorption lines are associated with outflowing gas of lower density. The polarization of both the transmitted and the reflected radiation is next computed for simple kinematic models of the outflow and the observed integrated polarization in the absorption-line troughs is found to be typically ∼ 10 per cent. An equatorial flow model gives a large degree of polarization (∼0.15) parallel to the symmetry axis in the absorption trough for the doublet transition J = 1/2→1/2, 3/2, and the polarized flux is found to extend to the red side. In contrast, we obtain a smaller degree of polarization ( ∼ 0.05) perpendicular to the jet axis from a bipolar flow model and the polarized flux is concentrated to the blue side of the line profile. It is predicted that the troughs of singlet J=0 →1 lines, such as C III λ 977, should exhibit larger degrees of polarization if this is due to resonance scattering. Polarization observations of quasar emission lines promise to be a powerful diagnostic of the kinematics of gas in the central pc of a quasar.