The Polarized Spectrum of the Fe [CSC]ii[/CSC]–Rich Broad Absorption Line QSO IRAS 07598+6508

Abstract

We present IUE spectrophotometry and optical spectropolarimetry of the ultraluminous, extreme Fe II-emitting QSO IRAS 07598+6508. We find broad absorption troughs from high- and low-ionization species, showing that this object is a member of the class of rare low-ionization broad absorption line (BAL) QSOs. Compared with non-BAL QSOs, the spectral energy distribution is reddened by E(B - V) ~ 0.12, and the Hα/Hβ ratio is even more reddened with E(B - V) ~ 0.45. The broad emission lines are unpolarized. We see broad Na I λ5892 absorption in the unpolarized continuum, but not in the polarized continuum (at the 5-6 σ level). The polarized continuum rises smoothly toward shorter wavelengths with Fλ ∝ λ-2. We argue that a QSO continuum is polarized by scattering from a region within, or very near, the broad emission line region (BELR). Thus there are at least three distinct light paths to the observer: a dusty path from the BELR; a direct path traced by the unpolarized continuum, passing through dust and low-ionization gas (Na I); and another relatively unobscured path followed by scattered continuum. This provides direct evidence that a BAL region and dust only partially cover the central QSO. Ultraluminous active galactic nuclei, including IRAS 07598+6508, appear no more IR-luminous than non-IRAS-selected QSOs, and have LIR/Lopt ratios when the optical luminosities are corrected for reddening. BALs and reddening occur only along some sight lines, and the parent population of BAL QSOs appear to be normal QSOs.