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Abstract

Our principal result is that the emission-line properties of non-BALQSOs and BALQSOs are remarkably similar. A comparison of the LBQS BALQSOs and non-BALQSOs reveals no statistically significant differences. However, when the full set of BALQSOs and various subsamples of the full set of BALQSOs are considered, we do find a small N V enhancement in the BALQSOs relative to the non-BALQSOs. We find enhanced emission around Al III λ1857, but comparison of the mean spectrum of the BALQSOs to the mean spectrum of the non-BALQSOs suggests that species other than Al III (probably Fe II and/or Fe III) are responsible for the difference. We also find a strong correlation between the strength of the Fe II emission and the balnicity index.

We define an index which measures the degree of detachment of the absorption trough. We find a weak anticorrelation between the detachment index and the equivalent widths of the C III] and C IV emission lines, but find no evidence of a correlation with this index and the Fe II strength. We confirm recent work by Corbin on velocity shifts between the peaks of Mg II, C III] and C IV: there is a small blueward displacement of the C IV peak relative to the C III] peak which is strongly correlated with the much larger blueward shift of C IV relative to Mg II. However, in contrast to Corbin's results, we do not find the shifts to be larger in the BALQSOs compared with the non-BALQSOs. There is evidence for structure in the C IV absorption troughs which has a velocity difference equal to that of the N V + Lyα velocity separation. This strongly suggests that radiation pressure is important in the dynamics of the outflow.

The continua of the BALQSOs and non-BALQSOs are remarkably similar, with the striking exception of the relatively rare BALQSOs which show strong Mg II and Al III absorption. These objects are much redder in the interval ˜1550 Å to ˜2200 Å than the other BALQSOs and non-BALQSOs. They also show especially strong Fe II emission, and evidence for Fe III emission much stronger than C III].

The close similarity between the emission-line and continuum properties of BALQSOs and non-BALQSOs is consistent with the view that BALQSOs do not form an intrinsically different class of objects from non- BALQSOs. We conclude that the small emission-line differences between non-BALQSOs and BALQSOs are more likely due to a difference in viewing angle, coupled with a mild anisotropy in the angular distribution of the flux of the emission lines, and a selection effect associated with a distribution of covering factors described by Morris. The Mg n-Al III BALQSOs may belong to an extreme population of objects where there is a large amount of obscuration present accompanying large, and therefore self-shielding, column densities, along with high electron densities.

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