CIV absorbers in z > 4 quasars: Tracing early galactic halos evolution

Abstract

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 Wrest(CIV) > 0.30 Å CIV doublets. We find that n(z) increases with time from z ∼ 4.5. In addition, Wrest(CIV) > 0.15 Å CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of Wrest(MgII)> 0.30 Å 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) ≥: 0.50 Å) 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.