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 alpha 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 alpha 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 a 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 proportional to (1 + z)(gamma). The number density of the low column density lines (N(HI) = (10(12.90) - 10(14.00)) cm(-2)) decreases with decreasing z with gamma = 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(13.64) cm(-2)) thin out faster (gamma = 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 beta = 1.60 +/- 0.03 over almost three orders of magnitude. beta is redshift independent. 3. The Ly alpha lines with lower column densities as well as the higher column density lines show marginal clustering with a 2 sigma significance over short distances (Delta v < 200 km s(-1) and Delta 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) over bar = (34 +/- 22) km s(-1). This value is typical for the analyzed region. It does not change significantly with z.