THE POLARIZATION AND ULTRAVIOLET-SPECTRUM OF MARKARIAN-231

Ultraviolet spectropolarimetry acquired with the Hubble Space Telescop e of the peculiar Seyfert galaxy Mrk 231 is combined with new high-qua lity ground-based measurements to provide the first, nearly complete, record of its linear polarization from 1575 to 7900 Angstrom. The acco mpanying ultraviolet spectrum portrays the heavily extinguished emissi on-line spectrum of the active nucleus plus the emergence of a blue co ntinuum shortward of similar to 2400 Angstrom. In addition, absorption features due to He I lambda 3188, Mg I lambda 2853, Mg II lambda 2798 , and especially several resonance multiplets of Fe II are identified with a well-known optical absorption system blueshifted similar to 460 0 km s(-1) with respect to the emission lines. The continuum is attrib uted to similar to 10(5) hot, young stars surrounding the nucleus. Thi s component dilutes the polarized nuclear light, implying that the int rinsic polarization of the active galactic nucleus (AGN) spectrum appr oaches 20% at 2800 Angstrom. The rapid decline in degree of polarizati on toward longer wavelengths is best explained by the strongly frequen cy-dependent scattering cross section of dust grains coupled with mode st starlight dilution. Peculiar S-shaped inflections in both the degre e and position angle of polarization through H alpha and other major e mission lines are interpreted as effects of scattering from two region s offset in velocity by several hundred km s(-1). A third source of (w eakly) polarized flux is required to explain a nearly 40 degrees rotat ion in position angle between 3200 and 1800 Angstrom. The displaced ab sorption features, polarimetry, and optical/infrared properties of Mrk 231 all point to its classification as a low-ionization, or ''Mg II'' broad absorption line quasar, in which most, if not all, lines of sig ht to the active nucleus are heavily obscured by dust and low-ionizati on gas clouds.