We have studied the metallicity of 17 high column density forests at /Vm = 1016~17 cm"2 and 2 <. z < 3 5 toward 7 high-z QSOs taken with the VLT/UVES, in order to study the metallicity of absorption line systems as a function of HI column density and z. This relation provides an important constraint on metal enrichment mechanisms of the intergalactic medium as well as early star formation in galaxies. We used higher order Lyman series to measure a reliable HI column density for the saturated Lya lines, and applied an ionisation correction assuming the Haardt-Madau UV background including both galaxies and QSOs. We have found 3, 6, and 8 systems with no metals, C IV-only, and additional ions other than CIV respectively, A photoionisation model with the Haardt-Madau background generally reproduces the observed metal line ratios. The median [C/H] is -3.03 and -1.96 for the 6 Civ-only systems and the 8 additional-ion systems respectively The inferred Hi number density is tig, ~ 10"3 5 cm"3 and ~ 10"2 5 cm"3 for the Civ-only systems and the additional-ion systems respectively When other ions are present, [C/H] of the high-JVHi forest is similar to that of sub-damped Lya systems.
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