The evolution of the physical state of the IGM^*

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. München, Germany e-mail: sdodoric@eso.org
² ST European Coordinating Facility, ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching b. München, Germany e-mail: scristia@eso.org
³ Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy

Corresponding author: T.-S. Kim, tkim@eso.org

Received: 25 August 2001
Accepted: 14 December 2001

Abstract

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 , , of the Lyα forest has been derived at three redshifts 2.1, 3.3 and 3.8. In particular, five QSOs at 2.1 enable us to study the cosmic variance of at lower z for the first time. When incompleteness of the number of the observed lines towards lower is accounted for, the derived slopes of , , are consistent with no-z evolution with an indication of lower value at 3.3, while at a fixed column density , , increases as z decreases. Assuming a QSO-dominated UV background, the slope of the equation of state 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 and even at the similar redshifts, in particular at 3.3 and 3.8. The lower and higher values at and 3.6 compared to ones at and 3.9 are caused by a lack of lower- and lower-b lines at lower-z parts of each QSO at , probably due to the reionization. This result suggests that an impact from the reionization on the forest might be mainly on the lower- forest. From this new dataset, we find some forest clouds with a high ratio of column density to column density, /, at , although the bulk of the forest clouds shows lower /. This high / at suggests that some forest clouds are exposed to a soft UV background. This lack of strong discontinuity of / at at suggests that / might not be a good observational tool to probe the reionization and/or that the UV background might be strongly affected by local, high-z galaxies at .