2019MNRAS.488.4356B

Sub-damped Lyman α systems (subDLAs; H I column densities of 19.0 <= logN(H I) < 20.3) are rarely included in the cosmic H I census performed at redshifts z_abs >= 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 ΔX = 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 Ly α 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 < z < 5, and thus have a small but significant contribution to the H I budget of the Universe.