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We investigate the evolution and the statistical properties of the Lyaabsorbers of the intergalactic medium (IGM) in the largely unexplored redshiftrange 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. Themain results for the combined Lya line sample are summarized as follows: 1. Theevolution of the number density of the absorbers can be described by the powerlaw dn/dz ~ (1+z)^gamma. The number density of the low column density linesdecreases with decreasing z with gamma=0.74+-0.31 in the interval z=0.7-1.9. Acomparison with results at higher redshifts shows that it is decelerated in theexplored redshift range and turns into a flat evolution for z -> 0. Thestronger absorbers thin out faster (gamma=1.50+-0.45). The break in theirevolution predicted for z=1.5-1.7 cannot be seen down to z=0.7. On the otherhand, a comparison with values from the literature for the local number densitygives a hint that this break occurs at lower redshift. 2. The distribution ofthe 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 beta=1.60+-0.03over almost three orders of magnitude. beta is redshift independent. 3. The Lyalines with lower column densities as well as the higher column density linesshow marginal clustering with a 2 sigma significance over short distances(Delta v < 200 km/s and Delta v < 100 km/s, respectively). We do not see anydifference in the clustering with either column density or redshift.Comment: 15 pages, 18 figures, accepted for publication in Astronomy &  Astrophysic

The evolution of Lyman α absorbers in the redshift range $0.5<\textit{z}<1.9$