The evolution of HI and CIV quasar absorption line systems at 1.9 < z < 3.2.

We have investigated the distribution and evolution of ~3100 intergalactic neutral hydrogen (HI) absorbers with HI column densities log N_HI = [12.75, 17.0] at 1.9 < z < 3.2, using 18 high resolution, high signal-to-noise quasar spectra obtained from the ESO VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one including all the available high-order Lyman lines to obtain reliable HI column densities of saturated lines, and another using only the Lyα transition. There is no significant difference between the Lyα-only fit and the high-order Lyman fit results. Combining our Lyα-only fit results at 1.7 < z < 3.6 with high-quality literature data, the mean number density at 0 < z < 4 is not well described by a single power law and strongly suggests that its evolution slows down at z ⩽ 1.5 at the high and low column density ranges.We also divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the CIV-enriched forest, depending on whether HI lines are associated with CIV at log NCIV ⩾ 12.2 within a given velocity range. The entire HI column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N_HI ~ 15. At log N_HI ⩽ 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The CIV-enriched forest dominates at log N_HI ⩾ 15, with its distribution function as ∝N^~-1.45 _HI . However, it starts to flatten out at lower N_HI, since the enriched forest fraction decreases with decreasing N_HI. The deviation from the power law at log N_HI = [14, 17] shown in the CDDF for the entire Hi sample is a result of combining two different Hi populations with a different CDDF shape. The total Hi mass density relative to the critical density is Ω_HI ~ 1.6 × 10^-6 h^-1, where the enriched forest accounts for ~40% of Ω_HI. [ABSTRACT FROM AUTHOR]