Comparisons of the emission-line and continuum properties of broad absorption line and normal quasi-stellar objects

Abstract

We compare the emission-line and continuum properties of a set of 25 broad absorption line QSOs (BALQSOs) and 29 normal QSOs (i.e., non- BALQSOs), both taken from the Large Bright Quasar Survey (LBQS), whose candidates were identified using the Cambridge Automatic Plate Measuring Facility. The LBQS sample of 25 BALQSOs is augmented by an additional 17 BALQSOs taken from other sources. We define a balnicity index in order to separate the non-BALQSOs from the BALQSOs as objectively as possible, as well as to provide a measure of the strength of the broad absorption line features. The comparisons are made by defining indices for various features, measuring them for each object, and applying the Kolmogorov- Smirnov (K-S) test to various BALQSO and non-BALQSO samples. We also compare the BALQSOs and non-BALQSOs by creating and comparing mean spectra for various samples. 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 J1857, 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 CIII] 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 in 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 A to ~2200 A 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 II-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.