The clustering properties of the Lyman alpha clouds

We analyse the clustering properties of a high-resolution (similar to 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 (Delta v less than or similar to 300 km s(-1)) with amplitude and significance increasing with increasing column density. The correlation scale at z similar to 3 turns out to be 200-300 h(50)(-1) 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 observation 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 N-HI greater than or similar to 14 and the haloes of protogalactic systems.