The Lyα forest at 1.5<z<4

Using high resolution (R similar to 45 000), high S/N ( similar to 20-50) VLT/UVES data, we have analyzed the Ly alpha forest of 3 QSOs in the neutral hydrogen (H I) column density range N-HI = 10(12.5-16) cm(-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 Ly alpha 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 Ly alpha forest shows little evolution in the column density range N-HI = 10(12.5-14) cm(-2), f(N-HI), of the Ly alpha forest shows little evolution in the increase of beta to beta similar to 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 > 10(14) cm(-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(13.64-16) cm(-2) is well fitted by a single power law, dn/dz proportional to (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 similar to 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(13.5) cm(-2), b(c,13.5), shows a weak increase with decreasing z, with a possible local b(c,13.5) maximum at z <similar to> 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 H I opacity, <(<tau>)over bar> H I, is well approximated by an empirical power law, <(<tau>)over bar> H I proportional to (1 + z)(3.34+/-0.17), at 1.5 < z < 4; 6. The baryon density, Omega (b), derived both from the mean H I 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, Omega, as a function of z.