Ly$α$ forest and the total absorption cross-section of galaxies -- an example of the NTT SUSI Deep Field

By extrapolating the accumulated low-redshift data on the absorption radius of galaxies and its luminosity scaling, it is possible to predict the total absorption cross-section of the gas associated with collapsed structures in the universe at any given epoch. This prediction can be verified observationally through comparison with the well-known spatial distribution of the QSO absorption systems. In this way, it is shown that HDF, NTT SUSI Deep Field and other such data give further evidence for the plausibility of origin of the significant fraction of the Ly$\alpha$ forest in haloes of normal galaxies.