Astronomy & Astrophysics

Free access article

Issue		A&A Volume 373, Number 3, July III 2001
Page(s)		757 - 781
Section		Cosmology
DOI		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. Cristiani^{1, 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 $N_\ion = 10^ \mathrm{cm}^$ 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, $f(N_\ion)$ , of the Ly $\alpha$ forest shows little evolution in the column density range $N_\ion = 10^ \mathrm{cm}^$ , $f(N_\ion) \propto N_\ion^{-\beta}$ , 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 $N_\ion > 10^ \mathrm{cm}^$ 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 $N_\ion = 10^ \mathrm{cm}^$ 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 $N_\ion = 10^ \mathrm{cm}^$ , $b_{\rm c, \mathrm}$ , shows a weak increase with decreasing z, with a possible local $b_{\rm c, \mathrm}$ 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, $\overline{\tau}_\ion$ , is well approximated by an empirical power law, $\overline{\tau}_\ion$ $\propto (1+z)^{3.34 \pm 0.17}$ , 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.