The evolution of H i and C iv quasar absorption line systems at 1.9 < z < 3.2

Abstract

We have investigated the distribution and evolution of ∼3100 intergalactic neutral hydrogen (Hi) absorbers with Hi column densities log NHi = [12.75, 17.0] at 1.9 <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 <3.6 with high-quality literature data, the mean number density at 0 <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 <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 NHi ∼ 15. At log NHi ≤ 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The Civ-enriched forest dominates at log NHi ≥ 15, with its distribution function as ∼N?-1.45 Hi . However, it starts to flatten out at lower NHi, since the enriched forest fraction decreases with decreasing NHi. The deviation from the power law at log NHi = [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 ωHi ∼ 1.6 ×10-6 h-1, where the enriched forest accounts for ∼40% of ωHi. © 2013 ESO.