V. D'Odorico, G. Cupani, S. Cristiani, R. Maiolino, P. Molaro, M. Nonino, M. Centurión, A. Cimatti, S. di Serego Alighieri, F. Fiore, A. Fontana, S. Gallerani, E. Giallongo, F. Mannucci, A. Marconi, L. Pentericci, M. Viel, G. Vladilo
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 in the visual, and 37.5 and 53.5 km/s 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 (CDDF) 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, Omega_CIV, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor ~2 for z>~ 5.3. Considering only the stronger C IV lines (13.8 < log N(CIV) < 15), Omega_CIV gently rises by a factor of ~10 between z~ 6.2 and z~ 1.5 with a possible flattening toward z ~0. The increase is well fitted by a power law: Omega_CIV = (2+-1)x10^{-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 photo-ionization models. As already claimed by cosmological simulations, we find that C IV is a good tracer of the metallicity in the low density IGM gas at z ~ 5-6 while at z~3 it arises in gas with over-density delta~100.
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http://arxiv.org/abs/1306.4604
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