8. Conclusions

We have presented high-resolution spectra of the quasar PKS 2126-158.

The analysis of the Ly forest shows that:

1. The peak value of the Doppler parameter distribution is km s^-1. This value is large enough to be in agreement with a standard ionization model, but not too high ( km s^-1) to make the Ly clouds contribution to baryonic matter exceed the standard nucleosynthesis value (Hu et al. 1995).
2. The plot of the statistical sample of Ly lines in the plane shows no significant correlation between the two parameters.
3. The column density distribution shows a cutoff, due to incompleteness and blending, for , for higher values it is well described by a power-law with (Hu et al. 1995; Giallongo et al. 1996).
4. The number density of lines per unit redshift is in agreement with a standard power-law evolution of the type with (Giallongo et al. 1996).

Clustering properties of the Ly lines have been investigated in another paper (Cristiani et al. 1997). Ly of higher column density () are found to cluster significantly on scales around 100 km s^-1.

From the analysis of metal absorptions the following results have been obtained:

1. Two new C IV absorption systems have been detected at z=3.2165 and z=2.5537.
2. A mean metallicity of dex solar has been found using the metal systems at z=2.8195, z=2.9072 and z=2.9675. In order to make the column densities of the intervening systems compatible with realistic assumptions about the cloud sizes and the [Si/C] ratios, it is necessary to assume an increase of a factor 10 in the jump at 4 Ryd of the standard spectrum of the ionizing UV background (Haardt & Madau 1997).
3. Merging the present data with those obtained at comparable resolution for the two quasars Q0055-26 and Q0000-26, has provided a relative large sample (71 doublets) of C IV absorptions, complete down to a column density . The C IV column density distribution is well fitted by a single power-law, with .
4. The mode of the Doppler parameter distribution is km ^-1.
5. The clustering properties of the individual components of the C IV features have been investigated making use of a TPCF in the velocity space. A significant signal is obtained for scales smaller than 200-300 km s^-1, . The result is consistent with previous findings (Sargent et al. 1988; Heisler et al. 1989) when the increased resolution and s/n ratio of the present data is taken into account.
6. Correlations between total equivalent widths and number of components and between total equivalent widths and velocity spread of the individual C IV systems are observed. The two-point correlation functions for lower column density C IV absorption systems, recently estimated by Womble et al. (1997) and Songaila & Cowie (1996), show a weaker signal than the present data. These three pieces of evidence suggest a trend of decreasing clustering amplitude with decreasing column density. An analogous behaviour has been observed for Ly lines by Cristiani et al. (1997).

Acknowledgements

We thank P. Madau, C. Porciani and S. Savaglio for enlightening discussions. SC acknowledges the support of the ASI contract 95-RS-38 and of the TMR-network of the European Community "Galaxy Formation and Evolution''.