Triple-ionised carbon associated with the low-density neutral hydrogen gas at 1.7 < z < 3.3: the integrated NHi–NCiv relation

From the Voigt profile fitting analysis of 183 intervening {C IV} systems at 1.7 < z < 3.3 in 23 high-quality UVES/VLT and HIRES/Keck QSO spectra, we find that a majority of {C IV} systems (˜75 per cent) display a well-characterized scaling relation between integrated column densities of {H I} and {C IV} with a negligible redshift evolution, when column densities of all the {H I} and {C IV} components are integrated within a given ±150 km s-1 range centred at the {C IV} flux minimum. The integrated {C IV} column density N_{C IV,sys} increases with N_{H I,sys} at log N_{H I,sys} in [14, {{16}}] and log N_{C IV,sys} in [11.8, 14.0], then becomes almost independent of N_{H I,sys} at log N_{H I,sys} ≥ {{16}}, with a large scatter: at log N_{H I,sys} in [14, 22], log {N_{C IV,sys}} = [C1/log {N_{H I,sys}} + C2} ] + C3, with C1 = -1.90 ± 0.55, C2 = -14.11 ± 0.19 and C3 = 14.76 ± 0.17, respectively. The steep (flat) part is dominated by {Si IV}-free ({Si IV}-enriched) {C IV} systems. Extrapolating the N_{H I,sys}-N_{C IV,sys} relation implies that most absorbers with log N_{H I} ≤ 14 are virtually {C IV}-free. The N_{H I,sys}-N_{C IV,sys} relation does not hold for individual components, clumps or the integrated velocity range less than ±100 km s-1. This is expected if the line-of-sight extent of {C IV} is smaller than {H I} and N_{C IV,sys} decreases more rapidly than N_{H I,sys} at the larger impact parameter, regardless of the location of the {H I}+{C IV} gas in intergalactic medium filaments or in intervening galactic haloes.