Spectropolarimetry of first broad absorption line quasars

We present Keck spectropolarimetry of two rare low-ionization broad absorption line (BAL) quasars (QSOs), FIRST J084044.5+363328 and FIRST J155633.8+351758, that also exhibit narrow absorption lines from metastable excited levels of Fe II (''iron Lo-BALs''). These QSOs were discovered in optical follow-ups to a deep radio survey; FIRST J155633.8+351758 is radio-loud, the first BAL QSO so identified. FIRST J084044.5+363328 is highly polarized and exhibits many features found in other BAL QSOs. The continuum is approximate to 4% polarized near the 2000 Angstrom rest frame, falling to approximate to 2% at longer wavelengths, at a position angle of approximate to 50 degrees. The emission lines are unpolarized. The polarization rises to approximate to 8% in the low-ionization troughs of Mg II lambda 2800 and Al III lambda 1860. The polarization and its position angle vary in a complicated manner across the metastable Fe II absorption lines, which suggests that more than one mechanism is at work or that the system geometry is complex. FIRST J155633.8+351758 may be the most highly polarized BAL QSO known, and it exhibits other unusual polarization properties compared with those of other highly polarized BAL QSOs. The continuum is approximate to 13% polarized near the 2000 Angstrom rest frame, falling to approximate to 7% at longer wavelengths, at a position angle of 153 degrees. The emission lines are polarized like the continuum, but in the absorption troughs the polarization drops to zero. Currently available data cannot yet discriminate among the possible lines of sight to BAL QSOs (edge-on, pole-on, or random).