A sample of 908 C iv absorber components clumped in 188 systems outside the Lyman forest in the redshift range 1.6 \lesssim z \lesssim 4.4 have been identified in Keck HIRES spectra of nine QSOs .
These and corresponding lines of Si iv , C ii , Si ii and N v have been fitted with Voigt profiles to obtain column densities and C iv Doppler parameters .
The properties of the C iv absorbers are almost constant although their system velocity spreads tend to increase with decreasing redshift .
We find a mild increase in C iv comoving mass density with decreasing redshift with a mean \langle \Omega _ { \scriptsize \textrm { C~ { } { \sc iv } } } \rangle = ( 3.8 \pm 0.7 ) \times 10 ^ % { -8 } 
( 1 \sigma uncertainty limits ; spatially flat \Lambda CDM cosmology with \Omega _ { \Lambda } = 0.7 , \Omega _ { M } = 0.3 and h = 0.71 ) , in broad agreement with earlier work .
Corresponding values of C/H in the Lyman forest based on \Omega _ { b } from the CMB and ionization fractions from our data are [ C/H ] _ { \langle z \rangle = 4.0 } \geq - 3.11 ^ { +0.14 } _ { -0.19 } and [ C/H ] _ { \langle z \rangle = 2.1 } \geq - 2.64 ^ { +0.15 } _ { -0.22 } , suggesting a rise by a factor \sim 3 .
Relating the hydrogen mass density more directly to regions containing the C iv absorbers our values for [ C/H ] become \gtrsim - 2.2 at \langle z \rangle = 4.0 and \gtrsim - 2.0 at \langle z \rangle = 2.1 .
C iv absorber components exhibit strong clustering out to \Delta _ { v } \lesssim 300 km s ^ { -1 } but there is no clustering on any scale between systems .
We argue that for our sample the C iv clustering is entirely due to the peculiar velocities of gas present in the outer extensions of galaxies .
We find no change in the median column density ratio Si iv /C iv with redshift , particularly no large change near z = 3 , contrary to previous observations ; other ionic ratios vary continuously with redshift .
We show that these are only partial indicators of ionization state and remedy this by use of specific pairs of ionic ratios .
We demonstrate that the majority of absorbers are photoionized and find that at z \lesssim 2.65 QSOs dominate the ionization of the absorption systems whereas at z \gtrsim 3.4 an additional , dominant contribution from galaxies with specific spectral characteristics and high radiative escape fraction in the energy range 1–4 Ryd is required .
These results also indicate that [ Si/C ] = 0.0–0.4 fits the data well .
Between z = 2.65 and z = 3.4 there is evident transition in the ionization properties of the absorbers , with large scatter .
The UV spectral properties required for the galaxies are not reproduced by standard stellar population synthesis models .
We conclude that the heavy element absorbers at z \gtrsim 3.4 are located close to galaxies and irradiated dominantly by them , consistent with our independent conclusion from clustering properties .