EDP Sciences
Free access
Volume 383, Number 3, March I 2002
Page(s) 747 - 754
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20011812

A&A 383, 747-754 (2002)
DOI: 10.1051/0004-6361:20011812

The evolution of the physical state of the IGM

T.-S. Kim1, S. Cristiani2, 3 and S. D'Odorico1

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

(Received 25 August 2001 / Accepted 14 December 2001 )

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 $N_\ion{H}{i}$, $b_\mathrm{c}(N_\ion{H}{i})$, of the Ly $\alpha$ 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 $b_\mathrm{c}(N_\ion{H}{i})$ at lower z for the first time. When incompleteness of the number of the observed lines towards lower $N_\ion{H}{i}$ is accounted for, the derived slopes of $b_\mathrm{c}(N_\ion{H}{i})$, $(\Gamma-1)$, are consistent with no- z evolution with an indication of lower value at $<\!z\!> \,=$ 3.3, while $b_\mathrm{c}(N_\ion{H}{i})$ at a fixed column density $N_\ion{H}{i} = 10^{13.6} \ \mathrm{cm}^{-2}$, bc(13.6), increases as z decreases. Assuming a QSO-dominated UV background, the slope of the equation of state $(\gamma-1)$ 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 $(\Gamma-1)$ and bc(13.6) even at the similar redshifts, in particular at $<\!z\!> \,=$ 3.3 and 3.8. The lower $(\Gamma-1)$ and higher bc(13.6) values at $z \sim 3.1$ and 3.6 compared to ones at $z \sim 3.4$ and 3.9 are caused by a lack of lower- $N_\ion{H}{i}$ and lower- b lines at lower- z parts of each QSO at z > 3, probably due to the $\ion{He}{ii}$ reionization. This result suggests that an impact from the $\ion{He}{ii}$ reionization on the forest might be mainly on the lower- $N_\ion{H}{i}$ forest. From this new dataset, we find some forest clouds with a high ratio of $\ion{Si}{iv}$ column density to $\ion{C}{iv}$ column density, $N_\ion{Si}{iv}$/ $N_\ion{C}{iv}$, at z < 2.5, although the bulk of the forest clouds shows lower $N_\ion{Si}{iv}$/ $N_\ion{C}{iv}$. This high $N_\ion{Si}{iv}$/ $N_\ion{C}{iv}$ at z < 2.5 suggests that some forest clouds are exposed to a soft UV background. This lack of strong discontinuity of $N_\ion{Si}{iv}$/ $N_\ion{C}{iv}$ at $N_\ion{H}{i} = 10^{14-17}~\mathrm{cm}^{-2}$ at $z \sim 3$ suggests that $N_\ion{Si}{iv}$/ $N_\ion{C}{iv}$ might not be a good observational tool to probe the $\ion{He}{ii}$ reionization and/or that the UV background might be strongly affected by local, high- z galaxies at z < 3.

Key words: cosmology: observations -- quasars: general -- intergalactic medium

Offprint request: T.-S. Kim, tkim@eso.org

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© ESO 2002