2002A&A...383..747K

Using a new, increased dataset of 7 QSOs from VLT/UVES observations combined with one QSO from the literature, the minimum Doppler parameters as a function of neutral hydrogen column density N_HI, b_c(N_HI), of the Lyα forest has been derived at three redshifts <z≥2.1, 3.3 and 3.8. In particular, five QSOs at <z≥2.1 enable us to study the cosmic variance of b_c(N_HI) at lower z for the first time. When incompleteness of the number of the observed lines towards lower N_HI is accounted for, the derived slopes of b_c(N_HI), (Γ-1), are consistent with no-z evolution with an indication of lower value at <z≥3.3, while b_c(N_HI) at a fixed column density N_HI=10^13.6cm^–2, b_c(13.6), increases as z decreases. Assuming a QSO-dominated UV background, the slope of the equation of state (γ-1) shows no z-evolution within large uncertainties and the temperature at the mean density, T₀, decreases as z decreases at three redshift ranges. There is a large fluctuation of (Γ-1) and b_c(13.6) even at the similar redshifts, in particular at <z≥3.3 and 3.8. The lower (Γ-1) and higher b_c(13.6) values at z∼3.1 and 3.6 compared to ones at z∼3.4 and 3.9 are caused by a lack of lower-N_HI and lower-b lines at lower-z parts of each QSO at z>3, probably due to the HeII reionization. This result suggests that an impact from the HeII reionization on the forest might be mainly on the lower-N_HI forest. From this new dataset, we find some forest clouds with a high ratio of SiIV column density to CIV column density, N_SiIV/N_CIV, at z<2.5, although the bulk of the forest clouds shows lower N_SiIV/N_CIV. This high N_SiIV/N_CIV at z<2.5 suggests that some forest clouds are exposed to a soft UV background. This lack of strong discontinuity of N_SiIV/N_CIV at N_HI=10^{14–17}cm^–2 at z∼3 suggests that N_SiIV/N_CIV might not be a good observational tool to probe the HeII reionization and/or that the UV background might be strongly affected by local, high-z galaxies at z<3.