A high-resolution study of the evolution of the Lyman alpha forest in the redshift interval 0.9 < z < 1.7

Spectroscopy with HST/STIS Echelle and VLT/UVES of the bright QSO HE 0515-4414 (z_em =1.73, B = 15.0) offers for the first time the opportunity to study the Lyman alpha forest in the redshift range 0.9 < z < 1.7 at a resolution ~ 10 km/s. The number density evolution of the Lyman alpha lines is well described by the power law approach dn/dz = (dn/dz)_0 (1+z)^gamma. We derive gamma = 2.23 +/- 1.21 for the strong lines (13.64 < log N_HI < 16.00), in agreement with the Lyman alpha forest evolution for z > 1.7. The expected slow-down in their evolution does not appear earlier than z ~ 1. For the weak lines (13.10 < log N_HI < 14.00) we find that the HE 0515-4414 data for z > 1 follow the trend with gamma = 1.40 known from z > 1.7 observations, i.e. we confirm the difference in evolution between weak and strong lines. We use the two-point velocity correlation function (TPCF) to search for clustering of the Lyman alpha lines, yet we detect no excess in the TPCF on scales up to 10,000 km/s.