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 ve locity space shows a significant signal on small velocity scales (Delt a v less than or similar to 300 km s(-1)) with amplitude and significa nce increasing with increasing column density. The correlation scale a t z similar to 3 turns out to be 200-300 h(50)(-1) kpc. A trend of inc reasing correlation with decreasing redshift is apparent. The existenc e of over- and underdensities on scales of a few tens of Mpc is confir med with a high confidence level and a number of possible structures a re 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 physi cal association between the Lyman alpha clouds with log N-HI greater t han or similar to 14 and the haloes of protogalactic systems.