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(H I) greater than or equal to 2 x 10(17) cm-(2) or C IV systems with W-r > 0.5 Angstrom) 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-alpha systems (N(H I) greater than or equal to 2 x 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 similar to3000 km s(-1) in 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 similar to1 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-alpha systems. In particular, for the damped system at z similar to 2.53788 in the spectrum of Q2344+1228, new estimates of the ratios O/Fe, C/Fe are obtained: 0.02 < [C/Fe] < 0.06, 0.06 < [O/Fe] < 0.2. They indicate that O and C are not over-solar in this system.