2004A&A...417..443P

We use 29 z>4 quasar spectra to build a homogeneous sample of high-redshift CIV absorbers. We use these data to calculate the number density, n(z), of W_rest(CIV)>0.30Å CIV doublets. We find that n(z) increases with time from z∼4.5. In addition, W_rest(CIV)>0.15Å CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of W_rest(MgII)>0.30Å MgII doublets decreases with time, in agreement with results from studies of LLS number densities. Below z<3, none of all the classes of absorbers show signs of evolution. We interpret this as the formation of galactic envelopes from smaller halos. Furthermore, the doublet ratio DR=W(CIV1548)/W(CIV1550) is found to decrease with time, a signature of the increase of the mean CIV column density. Finally, the W(SiIV1393)/W(CIV1548) ratios in strong absorbers (W_ rest_(CIV1548)≥0.50Å) is found to be approximately constant from z=3.5 to z=2.5 and then to decrease with time. This result suggests that the highest column density absorbers are not sensitive to changes in the ionization continuum.