The clustering properties of the Lyman alpha clouds

We analyse the clustering properties of a high-resolution (~10 km s^-1) sample of about 1600 Lyman alpha lines in the spectra of 15 quasars, obtained in the framework of an ESO key programme with the addition of literature data. The two-point correlation function in velocity space shows a significant signal on small velocity scales (Deltav<~300 km s^-1) with amplitude and significance increasing with increasing column density. The correlation scale at z~3 turns out to be 200-300 h^-1_50 kpc. A trend of increasing correlation with decreasing redshift is apparent. The existence of over- and underdensities on scales of a few tens of Mpc is confirmed with a high confidence level and a number of possible structures are identified. The present observations are found to be consistent with models of gravitationally induced correlations. A continuity scenario between Lyman alpha and metal systems emerges, with a suggested physical association between the Lyman alpha clouds with log NH_I>~14 and the haloes of protogalactic systems.