The Lyman-Alpha Forest at 1.5 < z < 4

Using high resolution (R ~ 45000), high S/N (20-50) VLT/UVES observations covering the Lyman-alpha forest at 1.5 < z < 2.4 combined with similar observations at z > 2.4 from the literature, we have presented the redshift evolution of the Lyman-alpha forest at 1.5 < z < 4. We applied two parallel analyses: the tradiational Voigt profile fitting and a statistical measure of the flux. The general conclusions from both analyses are in good agreement. In general, the properties of the Lyman-alpha forest at 1.5 < z < 2.4, such as the different density distribution function, the correlation strengths and the mean neutral hydrogen opacity, evolve as expected extrapolating from at z > 2.4. These results suggest the main drive of the redshift evolution of the forest at 1.5 < z < 4 is the Hubble expansion. When combined with the HST observations from the QSO key project at z < 1.7, a flattening of the line number density evolution does seem to occur at z ~ 1.2. The result suggests that the ionizing UV background might not evolve as strongly as a QSO-dominated background, indicating a contribution to the UV background from galaxies at z < 2. The lower cut-off Doppler paramter at the fixed column denisty log N = 13.5 shows a slight increase as z decreases, with a possible local maximum at z ~ 2.9. The slope of the lower cut-off Doppler parameter is not well-defined with z. However, it is flatter at z < 3.1 than at z ~ 3.75.