Sub-damped Lyman α systems in the XQ-100 survey I - Identification and contribution to the cosmological HI budget

Sub-damped Lyman α systems (subDLAs; H I column densities of 19.0 ≤ logN(H I) &lt;20.3) are rarely included in the cosmic H I census performed at redshifts z<SUB>abs</SUB> ≳ 1.5, yet are expected to contribute significantly to the overall H I mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path length ∆X = 475) in the XQ-100 legacy survey to investigate the contribution of subDLAs to the H I mass density of the Universe. The impact of X-Shooter's spectral resolution on Lyα absorber identification is evaluated, and found to be sufficient for reliably finding absorbers down to a column density of logN(H I)≥18.9. We compared the implications of searching for subDLAs solely using H I absorption versus the use of metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap-Monte Carlo simulation, we computed the column density distribution function (f(N, X)) and the cosmological H I mass density (Ω _{H I}) of subDLAs and compared with our previous work based on the XQ-100 damped Lyman α systems. We do not find any significant redshift evolution in f(N, X) or Ω _{H I} for subDLAs. However, subDLAs contribute 10-20 per cent of the total Ω _{H I} measured at redshifts 2 &lt; z &lt; 5, and thus have a small but significant contribution to the H I budget of the Universe.