Superclustering of damped Lyman-alpha absorption systems at redshifts above two

Observations of two high redshift quasi-stellar objects (QSOs) (z=3.23 and z=3.18) separated by 8 arcmin on the sky are presented. Two strong Lyman-α absorption systems are seen in each QSO, and in both QSOs, one absorber is at a redshift of 2. 380 and the other is at a redshift of 2.853. The velocities of the absorption systems differ between the lines of sight by only ∼300 km s −1 in both cases. To cover both lines of sight, the absorbers would need a transverse comoving size of ∼6h 100 −1 Mpc (for q 0 = 0. 5). The neutral hydrogen column densities inferred from the profiles lie between 1.2 × 10 18 and 1.2 × 10 20 cm −2 , and low ionization metal lines are associated with at least one of the absorption systems. The a posteriori joint probability of seeing these redshift coincidences is very low ∼3 × 10 −4