Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT

We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ~ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C IV doublet absorption systems. Six objects were observed with resolutions ~27 and 34 km s-1 in the visual, and 37.5 and 53.5 km s-1 in the near-infrared. We detected 102 C IV lines in the range: 4.35 < z < 6.2 of which 27 are above z ~ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si IV doublets associated with the C IV at z >= 5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z >= 5.3, with respect to the normalization defined by low-redshift (1.5 < z < 4) C IV lines. This behaviour is reflected in the redshift evolution of the C IV cosmic mass density, Ω _{C IV}, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of ~2 for z >= 5.3. Considering only the stronger C IV lines (13.8 < log N(C IV) < 15), Ω _{C IV} 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: Ω _{C IV} = (2± 1)× 10^{-8} [(1+z)/4]^{-3.1± 0.1}. An insight into the properties of the C IV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C IV, Si IV and C II absorptions with the output of a set of CLOUDY photoionization models. As already claimed by cosmological simulations, we find that C IV 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 δ ~ 100.