High matter density peaks from UVES observations of QSO pairs: Correlation properties and chemical abundances

We study the transverse clustering properties of high matter density peaks as traced by high column density absorption systems (either Lyman limit systems characterized by $N($ $\ion{H}{i}$ $) \ge 2 \times 10^{17}$ cm-2 or $\ion{C}{iv}$ systems with $W_{\rm r} > 0.5$ Å) at redshifts between 2 and 3 with UVES spectra of two QSO pairs (UM680/UM681 at 56 arcsec angular separation and Q2344+1228/Q2343+1232 at 5 arcmin angular separation) and a QSO triplet (Q2139-4433/Q2139-4434/Q2138-4427 at 1, 7 and 8 arcmin angular separation). We find 3 damped Lyman-α systems ($N($ $\ion{H}{i}$ $) \ge 2 \times 10^{20}$ cm-2): 2 coinciding with strong metal systems in the nearby line of sight and 1 matching the emission redshift of the paired QSO; plus 7 Lyman limit systems: 4 forming two matching couples and 3 without a corresponding metal system within ~3000 km s-1in the coupled line of sight. In summary, we detect five out of ten matching systems within 1000 km s-1, indicating a highly significant overdensity of strong absorption systems over separation lengths from ~1 to 8 h-1 Mpc. The observed coincidences could arise in gas due to starburst-driven superwinds associated with a quasar or a galaxy, or gas belonging to large scale structures like filaments or sheets. We also determine chemical abundance ratios for three damped Lyman-α systems. In particular, for the damped system at $z\sim 2.53788$ in the spectrum of Q2344+1228, new estimates of the ratios O/Fe, C/Fe are obtained: $\rm -0.02< [C/Fe]<0.06$, $\rm -0.06< [O/Fe]