The metallicity of the high column density Lyα forest at 2<z<3.5

We have studied the metallicity of 17 high column density forests at N_H{I}}=10^15-17 cm^-2 and 2 toward 7 high-z QSOs taken with the VLT/UVES, in order to study the metallicity of absorption line systems as a function of H I 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 H I column density for the saturated Lyα 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 C IV 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 C IV-only systems and the 8 additional-ion systems respectively. The inferred H I number density is N_HI ∼ 10^-3.5 cm^-3 and ∼ 10^-2.5 cm^-3 for the C IV-only systems and the additional-ion systems respectively. When other ions are present, [c/h] of the high-N_HI forest is similar to that of sub-damped Lyα systems.