The kinematic signature of damped Lyman alpha systems: using the D-index to screen for high column density H i absorbers★

doi:10.1111/j.1365-2966.2008.14131.x

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Wiley InterScience

Monthly Notices of the Royal Astronomical Society

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Monthly Notices of the Royal Astronomical Society: Letters

Volume 392 Issue 3, Pages 998 - 1007

Published Online: 22 Dec 2008

A Journal of the Royal Astronomical Society

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The kinematic signature of damped Lyman alpha systems: using the D-index to screen for high column density H i absorbers^★

Sara L. Ellison ¹†, Michael T. Murphy ² and Miroslava Dessauges-Zavadsky ³

¹ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada ² Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H39, PO Box 218, Victoria 3122, Australia ³ Geneva Observatory, University of Geneva, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Correspondence to †E-mail: sarae@uvic.ca

^★ Based on observations made with European Southern Observatory Telescopes at the Paranal Observatories under programmes 078.A-0003(A) and 080.A-0014(B).

KEYWORDS

galaxies: high redshift • quasars: absorption lines

ABSTRACT

Using a sample of 21 damped Lyman alpha systems (DLAs) and 35 sub-DLAs, we evaluate the D-index from high-resolution spectra of the Mg iiλ 2796 profile. This sample represents an increase in the sub-DLA statistics by a factor of 4 over the original D-index sample. We investigate various techniques to define the velocity spread (Δv) of the Mg ii line to determine an optimal D-index for the identification of DLAs. The success rate of DLA identification is 50–55 per cent, depending on the velocity limits used, improving by a few per cent when the column density of Fe ii is included in the D-index calculation. We recommend the set of parameters that are judged to be most robust, have a combination of high DLA identification rate (57 per cent) and low DLA miss rate (6 per cent) and most cleanly separate the DLAs and sub-DLAs (Kolmogorov–Smirnov probability 0.5 per cent). These statistics demonstrate that the D-index is the most efficient technique for selecting low-redshift DLA candidates: 65 per cent more efficient than selecting DLAs based on the equivalent widths of Mg ii and Fe ii alone. We also investigate the effect of resolution on determining the N(H i) of sub-DLAs. We convolve echelle spectra of sub-DLA Lyα profiles with Gaussians typical of the spectral resolution of instruments on the Hubble Space Telescope and compare the best-fitting N(H i) values at both the resolutions. We find that the fitted H i column density is systematically overestimated by ∼0.1 dex in the moderate-resolution spectra compared to the best fits to the original echelle spectra. This offset is due to blending of nearby Lyα clouds that are included in the damping wing fit at low resolution.