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

Abstract

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 {minus tilde} 27 and 34 km s-1 in the visual, and 37.5 and 53.5 kms-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 {greater-than or approximate} 5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z {greater-than or approximate} 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, ωCIV, of lines with column density in the range 13.4 < logN(C IV) < 15, which is consistent with a drop of a factor of ~2 for z {greater-than or approximate} 5.3. Considering only the stronger C IV lines (13.8 < logN(C IV) < 15), ωCIV gently rises by a factor of ~10 between z {minus tilde} 6.2 and z {minus tilde} 1.5 with a possible flattening towards z ~ 0. The increase is well fitted by a power law: ωCIV = (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 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 δ ~ 100. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.