2001A&A...373..757K

Using high resolution (R∼45000), high S/N (∼20-50) VLT/UVES data, we have analyzed the Lyα forest of 3 QSOs in the neutral hydrogen (HI) column density range N_HI=10^12.5–16cm^–2 at 1.5<z<2.4. We combined our results with similar high-resolution, high S/N data in the literature at z>2.4 to study the redshift evolution of the Lyα forest at 1.5<z<4. We have applied two types of analysis: the traditional Voigt profile fitting and statistics on the transmitted flux. The results from both analyses are in good agreement: 1. The differential column density distribution function, f(N_HI), of the Lyα forest shows little evolution in the column density range N_HI=10^12.5–14cm^–2, f(N_HI)∝N_HI^–β, with β∼1.4-1.5 at 1.5<z<4 and with a possible increase of β to β∼1.7 at z<1.8. A flattening of the power law slope at lower column densities at higher z can be attributed to more severe line blending. A deficiency of lines with N_HI>10¹⁴cm^–2 is more noticeable at lower z than at higher z. The one-point function and the two-point function of the flux confirm that strong lines do evolve faster than weak lines; 2. The line number density per unit redshift, dn/dz, at N_HI=10^13.64–16cm^–2 is well fitted by a single power law, dn/dz∝(1+z)^2.19±0.27, at 1.5<z<4. In combination with the HST results from the HST QSO absorption line key project, the present data indicate that a flattening in the number density evolution occurs at z∼1.2. The line counts as a function of the filling factor at the transmitted flux F in the range 0<F<0.9 are constant in the interval 1.5<z<4. This suggests that the Hubble expansion is the main drive governing the forest evolution at z>1.5 and that the metagalactic UV background changes more slowly than a QSO-dominated background at z<2; 3. The observed cutoff Doppler parameter at the fixed column density N_HI=10^13.5cm^–2, b_c, 13.5_, shows a weak increase with decreasing z, with a possible local b_c, 13.5_ maximum at z∼2.9; 4. The two-point velocity correlation function and the step optical depth correlation function show that the clustering strength increases as z decreases; 5. The evolution of the mean HI opacity, <τ>_HI, is well approximated by an empirical power law, <τ>_HI∝(1+z)^3.34±0.17, at 1.5<z<4; 6. The baryon density, Ω_b, derived both from the mean HI opacity and from the one-point function of the flux is consistent with the hypothesis that most baryons (over 90%) reside in the forest at 1.5<z<4, with little change in the contribution to the density, Ω, as a function of z.