The Evolution of Lyman α 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 z em < 1.94. 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 dn dz ∝ (1 + z) γ. The number density of the low column density lines (N H i = (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 H i > 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 N HI = 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. It does not change significantly with z.