ULTRAVIOLET SPECTROPOLARIMETRY OF HIGH-REDSHIFT QUASARS WITH THE HUBBLE-SPACE-TELESCOPE

Ultraviolet spectropolarimetry of three bright high-redshift low-polar ization quasars (LPQs) was obtained with the Faint Object Spectrograph of the Hubble Space Telescope. Two of the quasars, PG 1634+706 and PG 2302+029, had polarizations p similar to 0.5%-1.0% throughout the ult raviolet, and showed no significant variation of polarization amplitud e or position angle with wavelength. PG 2302+029 was also marginally ( 2.4 sigma) circularly polarized in the optical continuum. For the high est redshift quasar, PG 1222+228 (Ton 1530), the polarization was meas ured down to rest wavelengths below 800 Angstrom. Although the continu um of PG 1222+228 was weakened by Lyman limit absorption from an inter galactic gas cloud, the polarization increased sharply from 1% to abou t 4.5%, a change of 4 sigma significance. This abrupt rise in polariza tion does not appear attributable to any known instrumental artifact. These UV polarizations were only slightly less than those previously o bserved for these same objects in the optical. The polarization spectr a were flat with a typical slope of the polarized flux pF(nu) proporti onal to nu(-0.8+/-0.5). Unlike the case of several high luminosity Sey fert 1 nuclei studied previously, polarization caused by scattering fr om dust grains does not provide the best fit to the polarization spect ra of these luminous quasars. The hypotheses that the polarization in these quasars is produced by transmission through aligned interstellar grains (in the Milky Way or the host galaxy), or by a synchrotron pow er-law component, appear to be ruled out. These observed spectra are c onsistent with a wavelength-independent polarization proportional to t he total nonstellar light or, possibly, to the contribution of the blu e thermal component. The polarization spectra have insufficient signal -to-noise to locate the scatterers with respect to the continuum sourc e and the much larger broad line region. A decrease in amplitude and r otation of the position angle of the polarization vector at the shorte st wavelengths, which could result from general relativistic effects n ear a spinning black hole, was not observed. In fact, in PG 1222+228, the polarization was observed to increase at the shortest wavelengths. The rise in polarization with frequency is so sharp that it cannot be due to any wavelength-independent polarizing mechanism at any radius in an accretion disk. Such a rise could be attributable, for example, to a relative increase in scattering opacity over absorption at higher frequencies.