Abstract
We apply a recently developed method for classifying broad absorption line quasars (BALQSOs) to the latest QSO catalogue constructed from Data Release 5 of the Sloan Digital Sky Survey. Our new hybrid classification scheme combines the power of simple metrics, supervised neural networks and visual inspection. In our view the resulting BALQSO catalogue is both more complete and more robust than all previous BALQSO catalogues, containing 3552 sources selected from a parent sample of 28,421 QSOs in the redshift range 1.7<z<4.2. This equates to a raw BALQSO fraction of 12.5%. In the process of constructing a robust catalogue, we shed light on the main problems encountered when dealing with BALQSO classification, many of which arise due to the lack of a proper physical definition of what constitutes a BAL. This introduces some subjectivity in what is meant by the term BALQSO, and because of this, we also provide all of the meta-data used in constructing our catalogue, for every object in the parent QSO sample. This makes it easy to quickly isolate and explore sub-samples constructed with different metrics and techniques. By constructing composite QSO spectra from sub-samples classified according to the meta-data, we show that no single existing metric produces clean and robust BALQSO classifications. Rather, we demonstrate that a variety of complementary metrics are required at the moment to accomplish this task. Along the way, we confirm the finding that BALQSOs are redder than non-BALQSOs and that the raw BALQSO fraction displays an apparent trend with signal-to-noise, steadily increasing from 9% in low signal-to-noise data, up to 15%.
Highlights
Broad absorption line quasars (BALQSOs) are a subclass of active galactic nuclei (AGN) exhibiting strong, broad and blue-shifted spectroscopic absorption features (Foltz et al 1990; Weymann et al 1991; Hewett & Foltz 2003; Reichard 2003b)
In a previous paper (Knigge et al 2008, hereafter Paper I), we showed that both the balnicity index (BI) and a more recently defined metric, the absorption index (AI; Trump et al 2006), are biased when selecting BALQSOs, the former being incomplete at the low-velocity end of the BALQSO distribution, and the latter suffering from significant contamination by objects with low-velocity absorption systems which may be unrelated to the higher velocity outflows
We find that 3205 quasi-stellar object (QSO) (11.3 per cent of the parent sample) are classified as BALQSOs by the BI metric (i.e. BI > 0 km s−1), and 3282 QSOs (11.5 per cent) are classified as BALQSOs by the learning vector quantization (LVQ) network alone
Summary
Broad absorption line quasars (BALQSOs) are a subclass of active galactic nuclei (AGN) exhibiting strong, broad and blue-shifted spectroscopic absorption features (Foltz et al 1990; Weymann et al 1991; Hewett & Foltz 2003; Reichard 2003b). The vast majority of BALQSOs are radioquiet (Stocke et al 1992, but see Brotherton, de Breuck & Schaefer 2006 for some counter examples), and there are subtle differences between their continuum and emission-line properties and those of “normal” (non-BAL) quasi-stellar object (QSOs) (Reichard 2003b). The resulting catalogue contains 3552 BALQSOs selected from a parent sample of 28 421 QSOs on the basis of absorption close to the C IV high-ionization emission line. This catalogue may be obtained from http://www.astro.soton.ac.uk/∼simo. We provide (at the same address) a catalogue of the meta-data, i.e. the data pertaining to the parent QSO sample and subsequently used in the compilation of our BALQSO catalogue, so that members of the scientific community wishing to compile their own BAL/non-BAL subsamples may readily do so
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