EDP Sciences
Free access
Volume 417, Number 2, April II 2004
Page(s) 443 - 450
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034213

A&A 417, 443-450 (2004)
DOI: 10.1051/0004-6361:20034213

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

C. Péroux1, P. Petitjean2, 3, B. Aracil2, M. Irwin4 and R. G. McMahon4

1  Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34131 Trieste, Italy
2  Institut d'Astrophysique de Paris, 98 bis Bld Arago, 75014 Paris, France
3  LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75 014 Paris, France
4  Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

(Received 20 August 2003 / Accepted 3 December 2003)

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  $W_{\rm rest}({\rm {C}{IV}})>0.30$ Å CIV doublets. We find that n(z) increases with time from $z\sim4.5$. In addition, $W_{\rm rest}({\rm {C}{IV}})>0.15$ Å CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of  $W_{\rm rest}$(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 ${\rm DR}=W({\rm {C}{IV}}1548)$/ $W({\rm {C}{IV}}1550)$ 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 ( $W_{\rm
rest}({\rm {C}{IV}}1548) \ge 0.50$ 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.

Key words: cosmology: observations -- galaxies: evolution -- galaxies: quasars: absorption lines -- galaxies: halos -- galaxies: high-redshift

Offprint request: C. Péroux, peroux@ts.astro.it

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