2002A&A...390...13D

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(HI)≥2x10¹⁷cm^–2 or CIV systems with W_r>0.5Å) 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(HI)≥2x10²⁰cm^–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 ∼3000km/s in the coupled line of sight. In summary, we detect five out of ten matching systems within 1000km/s, indicating a highly significant overdensity of strong absorption systems over separation lengths from ∼1 to 8h^–1Mpc. 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∼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.