Misc,
Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT
(2013)cite arxiv:1306.4604Comment: Paper submitted to MNRAS. Revised version after referee report. 39 pages, 9 Tables, 33 Figures.
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 ~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.
