Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-12-16
Astron.Astrophys. 417 (2004) 443-450
Astronomy and Astrophysics
Astrophysics
20 pages, 8 figures, accepted for publication in Astronomy & Astrophysics. A&A style file added to fit overseas paper format
Scientific paper
10.1051/0004-6361:20034213
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_rest(CIV)>0.30 A CIV doublets. We find that n(z) increases with time from z ~ 4.5. In addition, W_rest(CIV)>0.15 A CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of W_rest(MgII)>0.30 A 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 DR=W(CIV 1548)/W(CIV 1550) is found to decrease with time, a signature of the increase of the mean CIV column density. Finally, the W(SiIV 1393)/W(CIV 1548) ratios in strong absorbers (W_rest(CIV 1548) > 0.50 A) 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.
Aracil Bastien
Irwin Mike
McMahon Richard
Peroux Celine
Petitjean Patrick
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