The Lyα forest at ^*,**

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 Dipartimento di Astronomia dell'Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy

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

Received: 23 August 2000
Accepted: 1 May 2001

Abstract

Using high resolution (), high (~20-50) VLT/UVES data, we have analyzed the Lyα forest of 3 QSOs in the neutral hydrogen (Hi) column density range at . We combined our results with similar high-resolution, high data in the literature at to study the redshift evolution of the Lyα forest at . 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, , of the Lyα forest shows little evolution in the column density range , , with -1.5 at and with a possible increase of β to at . 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 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, ddz, at is well fitted by a single power law, dd, at . 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 . The line counts as a function of the filling factor at the transmitted flux F in the range are constant in the interval . This suggests that the Hubble expansion is the main drive governing the forest evolution at and that the metagalactic UV background changes more slowly than a QSO-dominated background at ; 3. The observed cutoff Doppler parameter at the fixed column density , , shows a weak increase with decreasing z, with a possible local maximum at ; 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, , is well approximated by an empirical power law, , at ; 6. The baryon density, , 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 , with little change in the contribution to the density, Ω, as a function of z.