ON THE POLARIZATION OF RESONANTLY SCATTERED EMISSION-LINES .3. POLARIZATION OF QUASAR BROAD EMISSION-LINES AND BROAD ABSORPTION-LINE TROUGHS

The contribution to the expected linear polarization of quasar broad e mission and absorption lines from resonance scattering is computed usi ng a Monte Carlo approach for specific, generic models. Attention is f ocused on the external illumination of the scattering region. The pola rization of the reflected component from an externally illuminated sla b with finite optical depth is first considered as a model of a single , dense, broad emission-line cloud. A polarization less than or simila r to 40 per cent (less than or similar to 10 per cent) is typically co mputed for J=0-->1 (J=1/2-->3/2) transitions. Integration over a popul ation of emission-line clouds typically reduces the observed polarizat ion by a factor similar to(0.1-0.3)q(BELR), when q(BELR) is the coveri ng factor for emission lines, while preserving the relative polarizati ons of different lines. Consequently, the low degree of polarization o bserved in the red wings of quasar emission lines limits the density o f low column density emission-line clouds. Broad absorption lines are associated with outflowing gas of lower density. The polarization of b oth the transmitted and the reflected radiation is next computed for s imple kinematic models of the outflow and the observed integrated pola rization in the absorption-line troughs is found to be typically simil ar to 10 per cent. An equatorial flow model gives a large degree of po larization (similar to 0.15) parallel to the symmetry axis in the abso rption trough for the doublet transition J=1/2-->1/2, 3/2, and the pol arized flux is found to extend to the red side. In contrast, we obtain a smaller degree of polarization (similar to 0.05) perpendicular to t he jet axis from a bipolar flow model and the polarized flux is concen trated to the blue side of the line profile. It is predicted that the troughs of singlet J=0-->1 lines, such as C III lambda 977, should exh ibit larger degrees of polarization if this is due to resonance scatte ring. Polarization observations of quasar emission lines promise to be a powerful diagnostic of the kinematics of gas in the central pc of a quasar.