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We have investigated the distribution and evolution of ~3100 intergalactic HIabsorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2,using 18 high resolution, high S/N quasar spectra obtained from the ESOVLT/UVES archive. We used two sets of Voigt profile fitting analysis, oneincluding all the available high-order Lyman lines to obtain reliable HI columndensities of saturated lines, and another using only the Ly-alpha lines. Thereis no significant difference between the results from the two fits. Combiningour results with literature data, the mean number density at 0 < z < 4 is notwell described by a single power law and strongly suggests that its evolutionslows down at z < 1.5 at the high and low column density ranges. We alsodivided our entire HI absorbers at 1.9 < z < 3.2 into two samples, theunenriched forest and the CIV-enriched forest, depending on whether HI linesare associated with CIV at log N(CIV) > 12.2 within a given velocity range. Theentire HI column density distribution function (CDDF) can be described as thecombination of these two well-characterised populations which overlap at logN(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing asimilar power-law distribution to the entire forest. The CIV-enriched forestdominates at log N(HI) > 15, with its distribution function proportional toN(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since theenriched forest fraction decreases with decreasing N(HI). The deviation fromthe power law at log N(HI) = [14, 17] shown in the CDDF for the entire HIsample is a result of combining two different HI populations with a differentCDDF shape. The total HI mass density relative to the critical density isOmega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40%of Omega(HI).

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