Abstract
We carefully reconsider the problem of classifying broad-absorption line quasars (BALQSOs) and derive a new, unbiased estimate of the intrinsic BALQSO fraction from the Sloan Digital Sky Survey (SDSS) DR3 quasi-stellar object (QSO) catalogue. We first show that the distribution of objects selected by the so-called 'absorption index' (AI) is clearly bimodal in log AI, with only one mode corresponding to definite BALQSOs. The surprisingly high BALQSO fractions that have recently been inferred from AI-based samples are therefore likely to be overestimated. We then present two new approaches to the classification problem that are designed to be more robust than the AI, but also more complete than the traditional 'balnicity index' (BI). Both approaches yield observed BALQSO fractions around 13.5 per cent, while a conservative third approach suggests an upper limit of 18.3 per cent. Finally, we discuss the selection biases that affect our observed BALQSO fraction. After correcting for these biases, we arrive at our final estimate of the intrinsic BALQSO fraction. This is fBALQSO= 0.17 ± 0.01 (stat) ± 0.03 (sys) with an upper limit of fBALQSO? 0.23 . We conclude by pointing out that the bimodality of the log AI distribution may be evidence that the BAL-forming region has clearly delineated physical boundaries.
Highlights
Broad-absorption line quasars (BALQSOs) are a subclass of active galactic nuclei (AGN) that exhibit strong, broad and blueshifted spectroscopic absorption features (Foltz et al 1990; Weymann et al 1991; Reichard et al 2003)
We will use two new methods to determine observed BALQSO fractions that are more robust than absorption index’ (AI)-based estimates and more complete than balnicity index’ (BI)-based ones
It is worth noting that even the estimate which includes AI = 0 km s−1 objects lies substantially below the 26 per cent BALQSO fraction suggested by Trump et al (2006) based on the number of quasistellar object (QSO) with AI > 0 km s−1
Summary
Broad-absorption line quasars (BALQSOs) are a subclass of active galactic nuclei (AGN) that exhibit strong, broad and blueshifted spectroscopic absorption features (Foltz et al 1990; Weymann et al 1991; Reichard et al 2003). These typically narrower features can be due to absorption at an intermediate redshift along the line of sight to the QSO, absorption within the host galaxy, or intrinsic absorption close to the QSO (including the so-called miniBALS and associated absorption features) whose origin remains poorly understood and could conceivably be linked to the BALs. It is extremely difficult to say if any particular QSO containing an ‘intermediate’-width absorption trough (1000 v 3000 km s−1) should be classified as a BALQSO or not. As expected, this unimodal twopopulation model for the AI distribution produces a bimodal distribution in log AI (Fig. 2, top right-hand panel) Since this double-exponential model imposes no low-AI cut-off at all on the subpopulation that dominates at high AIs, it allows us to set a useful upper limit on the size of this population (see Sections 5.3 and 6.4)
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