SUPERCLUSTERING OF DAMPED LYMAN-ALPHA ABSORPTION SYSTEMS AT REDSHIFTS ABOVE 2

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-alpha 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 approximately 300 km s-1 in both cases. To cover both lines of sight, the absorbers would need a transverse comoving size of approximately 6h100(-1) Mpc (for q0 = 0.5). The neutral hydrogen column densities inferred from the profiles lie between 1.2 X 10(18) and 1.2 X 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, approximately 3 X 10(-4). If the absorbers are clustered, the probability of seeing the wavelength matches is enhanced, but a 20 Mpc scale supercluster with an absorber overdensity of approximately 30 is required to give even a 5% probability of seeing the observed wavelength matches. If, alternatively, individual giant gas clouds cover both lines of sight, they must have masses in excess of 10(11) M.. Any evidence for approximately 5 Mpc structure at these redshifts poses challenge to bottom-up structure formation scenarios such as the standard Cold Dark Matter model.