On the selection of damped Lyman α systems using Mg II absorption at 2 〈 z<SUB>abs</SUB> 〈 4

The XQ-100 survey provides optical and near-infrared coverage of 36 blindly selected, intervening damped Lyman α systems (DLAs) at 2 〈 z<SUB>abs</SUB> 〈 4, simultaneously covering the Mg II doublet at λλ2796, 2803Å, and the Ly α transition. Using the XQ-100 DLA sample, we investigate the completeness of selecting DLA absorbers based on their Mg II rest-frame equivalent width (W<SUB>0</SUB>^{2796}) at these redshifts. Of the 29 DLAs with clean Mg II profiles, we find that six (20 per cent of DLAs) have W<SUB>0</SUB>^{2796} 〈 0.6 Å. The DLA incidence rate of W<SUB>0</SUB>^{2796} 〈 0.6 Å absorbers is a factor of ̃5 higher than what is seen in z ̃ 1 samples, indicating a potential evolution in the Mg II properties of DLAs with redshift. All of the W<SUB>0</SUB>^{2796} 〈 0.6 Å DLAs have low metallicities (-2.5 〈 [M/H] 〈 -1.7), small velocity widths (v<SUB>90</SUB> 〈 50 km s<SUP>-1</SUP>), and tend to have relatively low N(H I). We demonstrate that the exclusion of these low W<SUB>0</SUB>^{2796} DLAs results in a higher mean N(H I) which in turn leads to an ̃7 per cent increase in the cosmological gas density of H I of DLAs at 2 〈 z<SUB>abs</SUB> 〈 4; and that this exclusion has a minimal effect on the H I-weighted mean metallicity.