2025ApJ...985...64Y

As part of the HIERACHY program, we collect the high signal-to-noise ratio and high-spectral resolution optical spectra of 25 quasars at z ≈ 4-5 to constrain the C IV evolution at z ≈ 3-5. In this paper, we report a catalog of 626 (1263) C IV absorption systems (components) at z ≈ 3-5 with a 50% completeness column density of log(N_Civ/cm^–2) ≈ 12.3. The HIERACHY/MIKE C IV sample is one of the best C IV absorber samples optimized to study the intergalactic medium during the He II reionization epoch. Using 557 (1090) intervening absorption systems (components), we found the column density distribution function of C IV absorption systems with log(N_Civ/cm^–2) ≳ 12.3 has a broken power-law shape, with the turnover column density log$({N}_{\mathrm{crit}}/{\mathrm{cm}}^{-2})=13.3{5}_{-0.19}^{+0.20}$, which is close to or smaller than the detection limit of most literature samples. We also found that both comoving path length number density dn/dX and cosmic abundance Ω for C IV absorption systems with log(N_Civ/cm^–2) > 13.2 show an increase (at the 2.2σ and 1.4σ levels, respectively) from redshift z ≈ 5-3, while absorption systems with log(N_Civ/cm^–2) = 12.3-13.2 exhibit a constant dn/dX and Ω_Civ.