a r X i v :a s t r o -p h /0101005v 2 2 M a y 2001

A&A manuscript no.

(will be inserted by hand later)

ASTRONOMY

AND

ASTROPHY SICS

Send offprint requests to :T.-S.Kim ⋆

Based on public data released from the UVES commission-ing at the VLT/Kueyen telescope,ESO,Paranal,Chile.⋆⋆

Table A.1,Table A.2and Table A.3are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5).

are constant in the interval 1.51.5and that the meta-galactic UV background changes more slowly than a QSO-dominated background at z <2.

3.

The observed cutoffDoppler parameter at the fixed column density N H i =1013.5cm −2,b c,13.5,shows a weak increase with decreasing z ,with a possible local b c,13.5maximum at z ∼2.9.

4.The two-point velocity correlation function and the step optical depth correlation function show that the clustering strength increases as z decreases.

5.The evolution of the mean H i opacity,

τH i ∝(1+

z )3.34±0.17,at 1.5

6.

The baryon density,Ωb ,derived both from the mean H i opacity and from the one-point function of the flux is consistent with the hypothesis that most baryons (over 90%)reside in the forest at 1.5

Key words:Cosmology:observations –quasars:Ly αforest –quasars:individual HE0515–4414,HE2217–2818,J2233–606,HS1946+7658,Q0302–003,Q0000–263