Triple-ionized carbon associated with the low-density neutral hydrogen gas at 1.7 < z < 3.3: The integrated NHI-NCIV relation

Abstract

From the Voigt profile fitting analysis of 183 intervening CIV systems at 1.7 < z < 3.3 in 23 high-quality UVES/VLT and HIRES/Keck QSO spectra, we find that a majority of CIV systems (~75 per cent) display a well-characterized scaling relation between integrated column densities of HI and CIV with a negligible redshift evolution, when column densities of all the HI and CIV components are integrated within a given ±150 km s-1 range centred at the CIV flux minimum. The integrated CIV column density NCIV,sys increases with NHI,sys at log NHI,sys ∈ [14, 16] and log NCIV,sys ∈ [11.8, 14.0], then becomes almost independent of NHI,sys at log NHI,sys ≥ 16, with a large scatter: at log NHI,sys ∈ [14, 22], log, 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) CIV systems. Extrapolating the NHI,sys-NC IV,sys relation implies that most absorbers with log NHI = 14 are virtually CIV-free. The NHI,sys-NC 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-ofsight extent of CIV is smaller than HI and NCIV,sys decreases more rapidly than NHI,sys at the larger impact parameter, regardless of the location of the HI+CIV gas in intergalactic medium filaments or in intervening galactic haloes.