The Evolution of Lyman-alpha Absorbers in the Redshift Range 0.5 < z < 1.9

We investigate the evolution and the statistical properties of the Ly α absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range z = 0.5−1.9. We use high-resolution (R ≥ 30 000) UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with zem 10 13.64 cm −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. 2. The distribution of the column densities of the absorbers is complete down to NHI = 10 12.90 cm −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. 3. The Ly α lines with lower column densities as well as the higher column density lines show marginal clustering with a 2σ significance over short distances (∆ v< 200 km s −1 and ∆ v< 100 km s −1 , respectively). We do not see any difference in the clustering with either column density or redshift. 4. The distribution of the Doppler parameters has a mean value of b = (34± 22) km s −1 . This value is typical for the analyzed region.