HST Ultraviolet and Ground-based Optical Spectropolarimetry of IRAS QSOs: Dusty Scattering in Luminous AGN

We present UV and optical spectropolarimetry of two highly polarized IRAS-selected QSOs, IRAS 13349+2438 and the BALQSO IRAS 14026+4341. The polarization in both objects rises rapidly toward the blue, peaks near 3000 A in the rest frame and remains nearly constant for shorter wavelengths. The rest frame optical polarized flux density spectra also increase rapidly towards the blue, but then decrease dramatically below 3000 A. This distinctive wavelength dependence of polarized flux shows that the polarization is produced by dust scattering. As for many Seyfert, radio and Hyperluminous Infrared Galaxies (HIGs), the lower polarization of the weak [O III] 4959,5007 lines in IRAS 13349+2438 suggests that the scattering grains lie interior to, or mixed with the narrow line gas. We construct full radiative transfer models of these systems consisting of a dusty sphere of modest optical depth illuminated axisymmetrically from within by a powerlaw QSO spectrum. We show that this simple model successfully reproduces the qualitative polarization properties of the objects. Despite similarities to other IRAS-selected BALQSOs, our FOS spectropolarimetry of IRAS 13349+2438 does not reveal broad absorption lines. IRAS 14026+4341 has an Al III BAL in both scattered and total flux density. We discuss these two objects in terms of both orientation and evolutionary unified schemes for QSOs, BALQSOs and HIGs.