2018MNRAS.481.4940C

We present further results of a survey for absorption line systems in the spectra of four high redshift quasars (5.79 <= z_em <= 6.13) obtained with the ESO Very Large Telescope X-Shooter. We identify 36 C IV and 7 Si IV systems with a >= 5σ significance. The highest redshift C IV and Si IV absorbers identified in this work are at z = 5.80738 ± 0.00017 and z = 5.77495 ± 0.00038, respectively. We compute the comoving mass density of Si IV (Ω_ Si IV_) and find that it evolves from Ω_ Si IV_ = 4.3^+2.1_–2.1 ×10^–9 at <z>= 5.05 to Ω_ Si IV_ = 1.4^+0.6_–0.4 ×10^–9 at <z>= 5.66. We also measure Ω_ C IV_ = 1.6^+0.4_–0.1 ×10^–8 at <z>= 4.77 and Ω_ C IV_ = 3.4^+1.6_–1.1 ×10^–9 at <z>= 5.66. We classify our C IV absorber population by the presence of associated low- and/or high-ionization systems and compute their velocity width (Δv₉₀). We find that all C IV systems with Δv₉₀ > 200 km s^–1 have associated low-ionization systems. We investigate two such systems separated by 550 physical kpc along a line of sight, and find it likely that they are both tracing a multiphase medium where hot and cold gas is mixing at the interface between the circumgalactic medium and intergalactic medium. We further discuss the Mg II systems presented in a previous work and we identify 5 Si II, 10 Al II, 12 Fe II, 1 C II, 7 Mg I, and 1 Ca II associated transitions. We compute the respective comoving mass densities in the redshift range 2-6, as allowed by the wavelength coverage.