On the selection of damped Lyman α systems using Mg II absorption at 2 < zabs < 4

Abstract

The XQ-100 survey provides optical and near-infrared coverage of 36 blindly selected, intervening damped Lyman alpha systems (DLAs) at 2 < z(abs) < 4, simultaneously covering the MgII doublet at lambda lambda 2796, 2803 angstrom, and the Ly alpha transition. Using the XQ-100 DLA sample, we investigate the completeness of selecting DLA absorbers based on their MgII rest-frame equivalent width (W-0(2796) 0) at these redshifts. Of the 29 DLAs with clean MgII profiles, we find that six (20 per cent of DLAs) have W-0(2796) 0 < 0.6 angstrom. The DLA incidence rate of W-0(2796) 0 < 0.6 angstrom absorbers is a factor of similar to 5 higher than what is seen in z similar to 1 samples, indicating a potential evolution in the MgII properties of DLAs with redshift. All of the W-0(2796) 0 < 0.6 angstrom DLAs have low metallicities (-2.5 < [M/H] < -1.7), small velocity widths (v(90) < 50 km s(-1)), and tend to have relatively low N(H I). We demonstrate that the exclusion of these low W-0(2796) 0 DLAs results in a higher mean N(H I) which in turn leads to an similar to 7 per cent increase in the cosmological gas density of HI of DLAs at 2 < z(abs) < 4; and that this exclusion has a minimal effect on the H I-weighted mean metallicity.