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Issue A&A
Volume 373, Number 3, July III 2001
Page(s) 757 - 781
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20010650



A&A 373, 757-781 (2001)
DOI: 10.1051/0004-6361:20010650

The Ly$\alpha$ forest at $\mathsf{1.5 < {\vec z} < 4}$ ,

T.-S. Kim1, S. Cristiani1, 1, 1 and S. D'Odorico1

1  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching b. München, Germany
    e-mail: sdodoric@eso.org ST European Coordinating Facility, ESO, Karl-Schwarzschild-Strasse 2, 85748, Garching b. München, Germany
    e-mail: scristia@eso.org Dipartimento di Astronomia dell'Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy

(Received 23 August 2000 / Accepted 1 May 2001 )

Abstract
Using high resolution ( $R \sim 45 000$), high S/N (~20-50) VLT/UVES data, we have analyzed the Ly$\alpha$ forest of 3 QSOs in the neutral hydrogen (HI) column density range $N_\ion{H}{i} = 10^{12.5-16} \mathrm{cm}^{-2}$ at 1.5 < z < 2.4. We combined our results with similar high-resolution, high S/N data in the literature at z > 2.4 to study the redshift evolution of the Ly$\alpha$ forest at 1.5 < z < 4. We have applied two types of analysis: the traditional Voigt profile fitting and statistics on the transmitted flux. The results from both analyses are in good agreement: 1. The differential column density distribution function, $f(N_\ion{H}{i})$, of the Ly$\alpha$ forest shows little evolution in the column density range $N_\ion{H}{i} = 10^{12.5-14} \mathrm{cm}^{-2}$, $f(N_\ion{H}{i}) \propto N_\ion{H}{i}^{-\beta}$, with $\beta \sim 1.4$-1.5 at 1.5 < z < 4 and with a possible increase of $\beta$ to $\beta \sim 1.7$ at z < 1.8. A flattening of the power law slope at lower column densities at higher z can be attributed to more severe line blending. A deficiency of lines with $N_\ion{H}{i} > 10^{14} \mathrm{cm}^{-2}$ is more noticeable at lower z than at higher z. The one-point function and the two-point function of the flux confirm that strong lines do evolve faster than weak lines; 2. The line number density per unit redshift, dn/dz, at $N_\ion{H}{i} = 10^{13.64-16} \mathrm{cm}^{-2}$ is well fitted by a single power law, dn/d $z \propto (1+z)^{2.19 \pm 0.27}$, at 1.5 < z < 4. In combination with the HST results from the HST QSO absorption line key project, the present data indicate that a flattening in the number density evolution occurs at $z \sim 1.2$. The line counts as a function of the filling factor at the transmitted flux F in the range 0 < F < 0.9 are constant in the interval 1.5 < z < 4. This suggests that the Hubble expansion is the main drive governing the forest evolution at z > 1.5 and that the metagalactic UV background changes more slowly than a QSO-dominated background at z < 2; 3. The observed cutoff Doppler parameter at the fixed column density $N_\ion{H}{i} = 10^{13.5} \mathrm{cm}^{-2}$, $b_{\rm c, \mathrm{13.5}}$, shows a weak increase with decreasing z, with a possible local $b_{\rm c, \mathrm{13.5}}$ maximum at $z \sim 2.9$; 4. The two-point velocity correlation function and the step optical depth correlation function show that the clustering strength increases as z decreases; 5. The evolution of the mean HI opacity, $\overline{\tau}_\ion{H}{i}$, is well approximated by an empirical power law, $\overline{\tau}_\ion{H}{i}$ $\propto (1+z)^{3.34 \pm 0.17}$, at 1.5 < z < 4; 6. The baryon density, $\Omega_\mathrm{b}$, derived both from the mean HI opacity and from the one-point function of the flux is consistent with the hypothesis that most baryons (over 90% ) reside in the forest at 1.5 < z < 4, with little change in the contribution to the density, $\Omega$, as a function of z.


Key words: cosmology: observations -- quasars: absorption lines

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

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