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Classifying broad absorption line quasars: metrics, issues and a new catalogue constructed from SDSS DR5

Classifying broad absorption line quasars: metrics, issues and a new catalogue constructed from SDSS DR5
Classifying broad absorption line quasars: metrics, issues and a new catalogue constructed from SDSS DR5
We apply a recently developed method for classifying broad absorption line quasars (BALQSOs) to the latest quasi-stellar object (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 per cent.

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 subsamples constructed with different metrics and techniques. By constructing composite QSO spectra from subsamples 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 ratio steadily increasing from 9 per cent in low signal-to-noise ratio data up to 15 per cent.
catalogues, surveys, quasars: absorption lines
1365-2966
2231-2238
Scaringi, S.
5a05d9eb-9bfb-455a-abb1-b1ed3229f977
Cottis, C. E.
758d0cec-1ce7-4408-a3fe-072257561729
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Goad, M. R.
9c241b66-bab0-487c-82c2-fe03162a9567
Scaringi, S.
5a05d9eb-9bfb-455a-abb1-b1ed3229f977
Cottis, C. E.
758d0cec-1ce7-4408-a3fe-072257561729
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Goad, M. R.
9c241b66-bab0-487c-82c2-fe03162a9567

Scaringi, S., Cottis, C. E., Knigge, C. and Goad, M. R. (2009) Classifying broad absorption line quasars: metrics, issues and a new catalogue constructed from SDSS DR5. Monthly Notices of the Royal Astronomical Society, 399 (4), 2231-2238. (doi:10.1111/j.1365-2966.2009.15426.x).

Record type: Article

Abstract

We apply a recently developed method for classifying broad absorption line quasars (BALQSOs) to the latest quasi-stellar object (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 per cent.

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 subsamples constructed with different metrics and techniques. By constructing composite QSO spectra from subsamples 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 ratio steadily increasing from 9 per cent in low signal-to-noise ratio data up to 15 per cent.

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More information

Published date: 9 September 2009
Keywords: catalogues, surveys, quasars: absorption lines

Identifiers

Local EPrints ID: 144163
URI: http://eprints.soton.ac.uk/id/eprint/144163
ISSN: 1365-2966
PURE UUID: 56221894-eefe-47e8-b928-7fb3f6ee0e8e

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Date deposited: 19 Apr 2010 09:16
Last modified: 14 Mar 2024 00:45

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Contributors

Author: S. Scaringi
Author: C. E. Cottis
Author: C. Knigge
Author: M. R. Goad

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