The evolution of H I and C IV quasar absorption line systems at 1.9 < z < 3.2

We have investigated the distribution and evolution of similar to 3100 intergalactic neutral hydrogen (H I) absorbers with H I 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 H I column densities of saturated lines, and another using only the Ly alpha transition. There is no significant difference between the Ly alpha-only fit and the high-order Lyman fit results. Combining our Ly alpha-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 H I absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the C IV-enriched forest, depending on whether H I lines are associated with C IV at log N-CIV >= 12.2 within a given velocity range. The entire H I column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N-HI similar to 15. At log N-HI <= 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The C IV-enriched forest dominates at log N-HI >= 15, with its distribution function as proportional to N-HI(similar to-1.45). 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 H I sample is a result of combining two different H I populations with a different CDDF shape. The total H I mass density relative to the critical density is O-HI similar to 1.6 x 10(-6) h(-1), where the enriched forest accounts for similar to 40% of Omega(HI).