Spectropolarimetry of Two Broad Absorption Line Quasars with the W. M. Keck Telescope

We have observed the broad absorption line quasars PHL 5200 and 0105265 with spectropolarimetry at the Keck telescope. In PHL 5200 the continuum is consistent with a power law with constant polarization, p 5.1%. A drop in polarization in the red is explained with dilution by unpolarized Fe II emission. In both objects the permitted emission lines are unpolarized. In PHL 5200 the semiforbidden line C III] 1909 is weakly polarized; we attribute this to resonance scattering. The polarization rises in the broad absorption troughs, to a peak of 12% in both objects. The high values of p are restricted to a narrow velocity range that is well inside the absorption troughs. In each object, the polarization position angle is constant, except that in PHL 5200 there are marginally significant rotations in the C IV trough and in the C III] emission line. We describe two simple geometries that can explain some of these features. In the first, there are scattered and direct rays; in the troughs the direct ray (unpolarized) is largely absorbed, and we mainly see the highly polarized scattered ray. In the other, the high trough polarization is caused by resonance scattering.