Abstract
We use 29 quasar spectra to build a homogeneous sample of high-redshift CIV absorbers. We use these data to calculate the number density, , of Å CIV doublets. We find that increases with time from . In addition, Å CIV systems are more numerous than the former at all redshifts. On the contrary, of Wrest(MgII) Å MgII doublets decreases with time, in agreement with results from studies of LLS number densities. Below , 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 / 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 ( Å) is found to be approximately constant from to and then to decrease with time. This result suggests that the highest column density absorbers are not sensitive to changes in the ionization continuum.
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