Hubble Space Telescope Ultraviolet and Ground-based Optical Spectropolarimetry of IRAS Quasi-stellar Objects: Dusty Scattering in Luminous Active Galactic Nuclei

Hines, Dean C., Schmidt, Gary D., Gordon, Karl D., Smith, Paul S., Wills, Beverley J., Allen, Richard G., & Sitko, Michael L.
2001, The Astrophysical Journal, 563, 512


We present UV and optical spectropolarimetry of two highly polarized IRAS-selected quasi-stellar objects (QSOs), IRAS 13349+2438 and the broad absorption line QSO (BALQSO) IRAS 14026+4341. The polarization in both objects rises rapidly toward the blue, peaks near 3000 Å in the rest frame and remains nearly constant for shorter wavelengths. The rest frame optical polarized flux density spectra also increase rapidly toward the blue but then decrease dramatically below 3000 Å. 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] λλ4959, 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 power-law 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 faint object spectrograph spectropolarimetry of IRAS 13349+2438 does not reveal broad absorption lines. IRAS 14026+4341 has an Al III broad absorption line 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. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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