The Polarization of Broad Absorption Line QSOs

Abstract

We present the combined results of optical polarization surveys of QSOs showing broad absorption lines (BALQSOs) conducted at the Steward and McDonald Observatories. The merged list of 53 objects provides the first statistical justification for claims of the tendency of BALQSOs to show stronger than average polarization, with a typical BALQSO being polarized a factor 2.4 times greater than a QSO from an optical survey selected without regard for absorption lines. Spectropolarimetry of sufficient quality to distinguish the polarization of emission lines versus absorption troughs versus continuum is also presented for six objects from the survey. When taken together with published data on other sources, the results lead to several significant correlations that can be used to gain insight into the structure of a BALQSO. These include: (1) a reduced degree of polarization in the broad emission lines, (2) enhanced polarization in the absorption troughs, and (3) an increase in the degree of polarization toward shorter wavelengths. In addition, BALQSOs with more prominent absorption systems tend to be more strongly polarized, and there is evidence that the subclass of low-ionization absorbers is more strongly polarized than other BALQSOs. If the increased polarization of BALQSOs is due to attenuation of direct, as opposed to scattered, lines of sight to the nucleus, absorption-line objects are underrepresented in optical surveys, and their true incidence is 20%-30% that of all UV-bright QSOs. Nevertheless, BALQSOs on average are not as highly polarized as their more obscured radio-quiet cousins, the IRAS QSOs/hyperluminous IR galaxies. We are led to a consistent picture in which broad absorption is observed in BALQSOs because they are inclined at intermediate inclinations, where our line of sight passes through gas clouds located near the surface of a dusty torus.