The evolution of the physical state of the IGM

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 NHi, bc(NHi), 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 bc(NHi) at lower z for the first time. When incompleteness of the number of the observed lines towards lower NHi is accounted for, the derived slopes of bc(NHi), (Γ-1), are consistent with no-z evolution with an indication of lower value at $<\!z\!> \,=$ 3.3, while bc(NHi) at a fixed column density NHi = 1013.6 \ cm⁻², bc(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 bc(13.6) even at the similar redshifts, in particular at $<\!z\!> \,=$ 3.3 and 3.8. The lower (Γ-1) and higher bc(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-NHi 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-NHi forest. From this new dataset, we find some forest clouds with a high ratio of Siiv column density to Civ column density, NSiiv/NCiv, at $z < 2.5$, although the bulk of the forest clouds shows lower NSiiv/NCiv. This high NSiiv/NCiv at $z < 2.5$ suggests that some forest clouds are exposed to a soft UV background. This lack of strong discontinuity of NSiiv/NCiv at NHi = 10¹⁴⁻¹⁷~cm⁻² at z ~ 3 suggests that NSiiv/NCiv 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 $z < 3$.