The polarization and ultraviolet spectrum of Markarian 231

Ultraviolet spectropolarimetry acquired with the Hubble Space Telescope of the peculiar Seyfert galaxy Mrk 231 is combined with new high-quality ground-based measurements to provide the first, nearly complete, record of its linear polarization from 1575 to 7900 Å. The accompanying ultraviolet spectrum portrays the heavily extinguished emission-line spectrum of the active nucleus plus the emergence of a blue continuum shortward of ∼2400 Å. In addition, absorption features due to He I λ3188, Mg I λ12853, Mg II λ12798, and especially several resonance multiplets of Fe n are identified with a well-known optical absorption system blueshifted ∼4600 km s^-1 with respect to the emission lines. The continuum is attributed to ∼ 10⁵ hot, young stars surrounding the nucleus. This component dilutes the polarized nuclear light, implying that the intrinsic polarization of the active galactic nucleus (AGN) spectrum approaches 20% at 2800 A. The rapid decline in degree of polarization toward longer wavelengths is best explained by the strongly frequency-dependent scattering cross section of dust grains coupled with modest starlight dilution. Peculiar S-shaped inflections in both the degree and position angle of polarization through Hα and other major emission lines are interpreted as effects of scattering from two regions offset in velocity by several hundred km s^-1. A third source of (weakly) polarized flux is required to explain a nearly 40° rotation in position angle between 3200 and 1800 Å. The displaced absorption features, polarimetry, and optical/infrared properties of Mrk 231 all point to its classification as a low-ionization, or "Mg H" broad absorption line quasar, in which most, if not all, lines of sight to the active nucleus are heavily obscured by dust and low-ionization gas clouds.