2002A&A...391L..11J

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 α forest in the redshift range 0.9<z<1.7 at a resolution ≤10km/s. The number density evolution of the Lyman α lines is well described by the power law approach dn/dz ∝(1+z)^γ. We derive γ=2.23±1.21 for the strong lines (13.64≤logN_HI≤16.00), in agreement with the Lyman α 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≤logN_HI≤14.00) we find that the HE 0515-4414 data for z>1 follow the trend with γ=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 α lines, yet we detect no excess in the TPCF on scales up to 10000km/s.