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