THE POLARIZED SPECTRUM OF THE Fe II-RICH BROAD ABSORPTION LINE QSO IRAS 07598+6508

摘要

We present WE 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 F_λ ∝ λ~(-2). We argue that a normal 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-lummous than non-IRAS-selected QSOs, and have normal L_(IR)/L_(opt) ratios when the optical luminosities are corrected for reddening. BALs and reddening occur only along some sight lines, and the parent population of HAL QSOs appear to be "normal" QSOs.