The evolution of Lyman αabsorbers 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 . We use high-resolution () UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with . The Ly α 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 Ly α line sample are summarized as follows: 1. The evolution of the number density of the absorbers can be described by the power law . The number density of the low column density lines ( cm-2) decreases with decreasing z with in the interval . A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for . The stronger absorbers ( cm-2) thin out faster (). The break in their evolution predicted for cannot be seen down to . 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 cm-2. It can be approximated by a single power law with the exponent 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 significance over short distances ( km s-1 and 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 km s-1. This value is typical for the analyzed region. It does not change significantly with z.