Triple-ionised carbon associated with the low-density neutral hydrogen gas at 1.7 < z < 3.3: the integrated N(HI)-N(CIV) relation

TL;DR

This study analyzes the relationship between neutral hydrogen and triply ionized carbon in the intergalactic medium at redshifts 1.7 to 3.3, revealing a consistent scaling relation with implications for the distribution of CIV and HI absorbers.

Contribution

It provides the first detailed characterization of the integrated N(HI)-N(CIV) relation across a large sample of CIV systems at high redshift, highlighting the relation's form and scatter.

Findings

Most CIV systems (~75%) follow a well-characterized N(HI)-N(CIV) relation.

The relation flattens at high N(HI), indicating CIV saturation or depletion.

Most low N(HI) absorbers are nearly CIV-free.

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%) display a well-characterised 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, +150) km/sec range centred at the CIV flux minimum. The integrated CIV column density N(CIV, sys) increases with N(HI, sys) at log N(HI, sys) = 14.0--15.5 and log N(CIV, sys) = 11.8--14.0, then becomes almost independent of N(HI, sys) at log N(HI, sys) > 16, with a large scatter: at log N(HI, sys) = 14--22, log N(CIV, sys) = C1 / (log(NHI, 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 SiIV-free (SiIV-enriched) CIV systems. Extrapolating the N(HI, sys)-N(CIV, sys) relation implies that most absorbers with log N(HI) < 14 are virtually CIV-free. The N(HI, sys)-N(CIV, sys) relation does not hold for individual components, clumps or the integration velocity range less than +-100 km/sec. It is expected if the line-of-sight extent of CIV is smaller than HI and N(CIV, sys) decreases more rapidly than N(HI, sys) at the larger impact parameter, regardless of the location of the HI+CIV gas in the IGM filaments or in the intervening galactic halos.