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Astron. Astrophys. 358, 835-840 (2000)

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1. Introduction

About 12% of optically-selected QSOs exhibit broad absorption lines (BALs) in their spectrum, that is resonance line absorption troughs that extend blueward of the emission lines up to [FORMULA]. Since the continuum and emission line properties of most BAL QSOs are not found to significantly differ from those of normal (non-BAL) QSOs, it is generally thought that rapidly moving absorbing matter exists in all (at least all radio-quiet) QSOs with a small covering factor, the BAL QSOs being those objects with the absorption region (the BALR) along the line of sight (Junkkarinen 1983, Weymann et al. 1991). Alternatively, if one focuses on the different behavior of the rarer objects with low-ionization absorption troughs, BAL QSOs could constitute a distinct population of QSO, possibly in a different evolutionary stage (Boroson & Meyers 1992). An important characteristic of BAL QSOs is the absence of powerful radio-sources among them (Stocke et al. 1992, Kuncic 1999).

The hypothesis that all radio-quiet QSOs are surrounded by a BALR implies that all differences between BAL and non-BAL QSOs are due to orientation. There are at least two important differences between BAL and non-BAL QSOs: as a class, BAL QSOs are more polarized than non-BAL QSOs (Stockman et al. 1984, Hutsemékers et al. 1998, Schmidt & Hines 1999), and they are more frequently found among radio-intermediate QSOs (Francis et al. 1993). To interpret these differences within the orientation -or unification- model, Goodrich (1997) has suggested that at least some BAL QSOs must have an attenuated direct continuum along our line of sight, such that the scattered light is more important in the total light we see, and then the observed polarization is larger than in non-BAL QSOs. Such an attenuation simultaneously explains why some BAL QSOs dominate the radio-intermediate class: the optical continuum is fainter, and the ratio of radio to optical fluxes is higher. This also means that the true fraction of BAL QSOs in optically-selected samples could be severely underestimated (Goodrich 1997).

The main goal of the present paper is to verify a direct consequence of this interpretation: the existence of a correlation between the optical continuum polarization of BAL QSOs and their radio-to-optical flux ratio. We have therefore compiled linear polarization measurements for a sample of BAL QSOs with known radio properties, essentially taken from the Stocke et al. (1992) VLA radio survey. Additional polarization data have been obtained for BAL QSOs with more extreme radio properties, including the five radio-loud BAL QSOs recently discovered by Brotherton et al. (1998). Furthermore, we investigate possible correlations between absorption line indices and radio properties, and more particularly the claimed anticorrelation between the terminal velocity of the flow and the radio-to-optical flux ratio (Weymann 1997). This relation, if confirmed, provides strong contraints on theoretical models, and may constitute a clue to the radio-loud / radio-quiet dichotomy in QSOs (Murray et al. 1995, Kuncic 1999).

The paper is organized as follows: in Sect. 2, we present the data, new measurements and compilation. The statistical analysis and the results are presented in Sect. 3. Discussion and conclusions form the last section.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000