2013A&A...552A..77K

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 N_CIV≥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_HI^~–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 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∼1.6x10^–6h^–1, where the enriched forest accounts for ∼40% of Ω_HI.