An efficient technique for pre-selecting low-redshift damped Lyα systems

The number of z similar to 1 damped Ly alpha systems (DLAs, log N(H i) >= 20.3) per unit redshift is approximately 0.1, making them relatively rare objects. Large, blind QSO surveys for low-redshift DLAs are therefore an expensive prospect for space-borne ultraviolet telescopes. Increasing the efficiency of these surveys by pre-selecting DLA candidates based on the equivalent widths (EWs) of metal absorption lines has previously been a successful strategy. However, the success rate of DLA identification is still only similar to 35 per cent when simple EW cut-offs are applied, the majority of systems having 19.0 < log N(H i) < 20.3. Here, we propose a new way to pre-select DLA candidates. Our technique requires high-to-moderate-resolution spectroscopy of the Mg II lambda 2796 transition, which is easily accessible from the ground for 0.2 less than or similar to z less than or similar to 2.4. We define the D-index, the ratio of the line equivalent width to velocity spread, and measure this quantity for 19 DLAs and eight sub-DLAs in archival spectra obtained with echelle spectrographs. For the majority of absorbers, there is a clear distinction between the D-index of DLAs compared with sub-DLAs (Kolmogorov-Smirnov probability = 0.8 per cent). Based on this pilot data sample, we find that the D-index can select DLAs with a success rate of up to 90 per cent, an increase in selection efficiency by a factor of 2.5 compared with a simple EW cut. We test the applicability of the D-index at lower resolution and find that it remains a good discriminant of DLAs for full width at half-maximum (FWHM) less than or similar to 1.5 angstrom. However, the recommended D-index cut-off between DLAs and sub-DLAs decreases with poorer resolution and we tabulate the appropriate D-index values that should be used with spectra of different resolutions.