z~6 QSOs CIV doublet absorption systems

This behaviour is reflected in the redshift evolution of the CIV cosmic mass density, {Omega}_CIV_, of lines with column density in the range 13.4<logN(CIV)<15, which is consistent with a drop of a factor of ~2 for z>~5.3. Considering only the stronger CIV lines (13.8<logN(CIV)<15), {Omega}_CIV_ gently rises by a factor of ~10 between z=~6.2 and z=~1.5 with a possible flattening towards z~0. The increase is well fitted by a power law: {Omega}_CIV=(2+/-1)x10^-8^[(1+z)/4]^-3.1+/-0.1^.

An insight into the properties of the CIV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated CIV, SiIV and CII absorptions with the output of a set of cloudy photoionization models. As already claimed by cosmological simulations, we find that CIV is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z~5-6 while at z~3 it arises in gas with overdensity {delta}~100.