| Abstract
| - Abstract. Combining a new, increased data set of eight quasi-stellar objects (QSOs) covering the Lyα forest at redshifts 1.5 < z< 3.6 from VLT/UVES observations with previously published results, we have investigated the properties of the Lyα forest at 1.5 < z<4. With the six QSOs covering the Lyα forest at 1.5 < z< 2.5, we have extended previous studies in this redshift range. In particular, we have concentrated on the evolution of the line number density and the clustering of the Lyα forest at z≤ 2.5, where the Lyα forest starts to show some inhomogeneity from sightline to sightline. We have fitted Voigt profiles to the Lyα absorption lines as in previous studies, and have, for two QSOs with zem∼ 2.4, fitted Lyα and higher order of Lyman lines down to 3050 Å simultaneously. This latter approach has been taken in order to study the Lyβ forest at z∼ 2.2 and the higher H i column density Lyα forest in the Lyβ forest region. For a given NH I range, the Lyα forest at 1.5 < z< 4 shows the monotonic evolution, which is governed mainly by the Hubble expansion at this redshift range. In general, the Lyα forest line number density (dn/dz) is best approximated with dn/dz= 6.1(1 +z)2.47 ± 0.18 for the H i column density NH I= 1013.64−17 cm−2 at 1.5 < z< 4. When the results at 0 < z< 1.5 from Hubble Space Telescope (HST) observations are combined, the slow-down in the number density evolution occurs at z< 1.5. For higher column density clouds at NH i> 1014 cm−2, there is a variation in the line number density from sightline to sightline at z< 2.5. This variation is stronger for higher column density systems, probably due to more gravitationally evolved structures at lower z. The mean H i opacity is at 1.5 < z< 4. HST observations show evidence for slower evolution of at z< 1. For NH i= 1012.5-15 cm−2, the differential column density distribution function, f(NH i), can be best fitted by f(NH I∝NH i−β with β≈ 1.5 for 1.5 < z< 4. When combined with HST observations, the exponent β increases as z decreases at 0 < z< 4 for NH i= 1013-17 cm−2. The correlation strength of the step optical depth correlation function shows the strong evolution from 〈z〉= 3.3 to 〈z〉= 2.1, although there is a large scatter along different sightlines. The analyses of the Lyβ forest at z∼ 2.2 are, in general, in good agreement with those of the Lyα forest.
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