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 ( \sim 75 % ) display a well-characterised 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 \pm 150 km s ^ { -1 } range centred at the C iv flux minimum .
The integrated C iv column density N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } increases with N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } at \log N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } \in [ 14 , 16 ] and \log N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } \in [ 11.8 , 14.0 ] , then becomes almost independent of N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } at \log N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } \geq 16 , with a large scatter : at \log N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } \in [ 14 , 22 ] , \log \mbox { $N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } $ } \ / = \left [ \frac { C _ { 1 % } } { \log \mbox { $N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } $ } \ / + C _ { 2 } } \right ] + % C _ { 3 } , with C _ { 1 } = -1.90 \pm 0.55 , C _ { 2 } = -14.11 \pm 0.19 and C _ { 3 } = 14.76 \pm 0.17 , respectively .
The steep ( flat ) part is dominated by Si iv -free ( Si iv -enriched ) C iv systems .
Extrapolating the N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } – N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } relation implies that most absorbers with \log N _ { \mathrm { { \mathrm { H \mathsc { i } } } } } \leq 14 are virtually C iv -free .
The N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } – N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } relation does not hold for individual components , clumps or the integrated velocity range less than \pm 100 km s ^ { -1 } .
This is expected if the line-of-sight extent of C iv is smaller than H i and N _ { \mathrm { { \mathrm { C \mathsc { iv } } } , sys } } decreases more rapidly than N _ { \mathrm { { \mathrm { H \mathsc { i } } } , sys } } at the larger impact parameter , regardless of the location of the H i +C iv gas in IGM filaments or in intervening galactic halos .