2006A&A...458..427J

We investigate the evolution and the statistical properties of the Lyα absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range z=0.5-1.9. We use high-resolution (R≥30000) UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with z_em<1.94. The Lyα lines detected in the lines of sight (LOS) towards these quasars are evaluated with a software package which determines simultaneously the quasar continuum and the line profiles. The main results for the combined Lyα line sample are summarized as follows: (o) The evolution of the number density of the absorbers can be described by the power law (dn)/(dz)∝(1+z)^γ. The number density of the low column density lines (N_HI=(10^12.90-10^14.00)cm^–2) decreases with decreasing z with γ=0.74±0.31 in the interval z=0.7-1.9. A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for z->0. The stronger absorbers (N_HI>10^13.64cm^–2) thin out faster (γ=1.50±0.45). The break in their evolution predicted for z=1.5-1.7 cannot be seen down to z=0.7. On the other hand, a comparison with values from the literature for the local number density gives a hint that this break occurs at lower redshift. (o) The distribution of the column densities of the absorbers is complete down to N_HI=10^12.90cm^–2. It can be approximated by a single power law with the exponent β=1.60±0.03 over almost three orders of magnitude. β is redshift independent. (o) The Lyα lines with lower column densities as well as the higher column density lines show marginal clustering with a 2σ significance over short distances (Δv<200km/s and Δv<100km/s, respectively). We do not see any difference in the clustering with either column density or redshift. (o) The distribution of the Doppler parameters has a mean value of <b≥(34±22)km/s. This value is typical for the analyzed region. It does not change significantly with z.