6 Conclusions

We have analysed new, high resolution UVES spectra of two QSO pairs and a QSO triplet (refer to Table 1 and Sect. 2) focussing mainly on the clustering properties of high matter density peaks, traced by LLS ( $2\times 10^{17} < N$(H I)/cm^-2 $< 2\times 10^{20}$) and DLAS (N(H I $) > 2\times 10^{20}$ cm^-2). The observed number of DLAS and LLS in the considered lines of sight is in good agreement with the expected value. The relevant conclusions are the following:

1.

in 4 cases out of 10 there is a metal system with C IV rest equivalent width $W_{\rm r} > 0.5$ Å, in the paired line of sight within 1000 km s^-1 of the redshift of the considered high column density absorption system. In 1 case, a DLAS matches the emitting QSO in the paired line of sight;

2.

the correlation signal is highly significant in spite of the small sample. The gas giving rise to the coincidences close to the emission redshift of the QSOs (# 2 and 3 in Fig. 15) could be due to a starburst-driven galactic superwind. In the other cases (# 5, 6 and 9), involving DLASs, the gas could be in coherent filamentary or sheet-like structures of several Mpc, the possible ancestors of present-day rich clusters;

3.

we measure the chemical abundance ratios in two DLAS and a sub-DLAS. In particular, we estimate the ratios C/Fe, O/Fe and Si/Fe for the DLAS at $z\sim 2.53788$ in the spectrum of Q2344+1228. The abundance ratios of these elements are consistent with solar values or very small enhancement at a variance with what is observed for halo stars at the same metallicity in the Milky Way.

Acknowledgements

V.D. is supported by a Marie Curie individual fellowship from the European Commission under the programme "Improving Human Research Potential and the Socio-Economic Knowledge Base'' (Contract No. HPMF-CT-1999-00029). This work was supported in part by the European Community RTN network "The Physics of the Intergalactic Medium''. It is a pleasure to thank C. Ledoux for the UVES spectrum of Q2138-4427.