The wavelength dependence of polarization of active galaxies and quasars

We measured UBV R1 polarizations of 0.5%-2.5% in more than a dozen "normal" quasars and Seyfert 1 galaxies. The position angle is wavelength-independent, suggesting the polarization in a given object originates in a single physical process. In many cases the percentage of polarization increases with frequency. By fitting the multiwavelength spectra with models, we estimate the fractions of the observed flux originating in separate continuum components: infrared, recombination, stellar, and thermal. We compare the polarized fluxes observed in each filter with the predictions of competing models of polarization in AGNs: synchrotron emission, scattering from electrons or different types of dust grains, and electron scattering in an accretion flow. In nine of the sources, the polarization is best understood as scattering off dust grains. Eight of these, 0134 + 329 (3C 48), 0711 + 458 (Mrk 376), 1004 + 130 (4C 13.41), 1426 + 015 (Mrk 1383), 1534 + 580 (Mrk 290), 1535 + 547 (Mrk 486), PG 1552 + 08, and PG 2209 + 185, appear to have a nonspherical distribution of dust with grains best described by the 1977 model of Mathis, Rumpl, and Norsieck. These objects tend to be redder and, in certain cases (e.g., Mrk 486), have broad emission lines which show the same polarization as the continuum. The second most successful fit to the data assumes polarization proportional to the flux of the blue thermal component. This model best characterizes 1208 + 397 (NGC 4151), 2041 - 109 (Mrk 509), and possibly 1415 + 253 (NGC 5548) and 1534 + 580 (Mrk 290). These objects have lower levels of polarization and unpolarized (where measured) emission lines. The modest polarization associated with the thermal ultraviolet component might be due to electron scattering in an accretion disk or torus.