Evolution of the ionization state of the intergalactic medium through observations of QSO metal line absorption systems

Several bulk properties of the metal absorption systems seen against background QSO are shown to be almost invariant over a wide redshift range. The absorber components also exhibit strong short-range clustering but there is no clustering on any scale between composite systems. At redshifts lower than 2.7 the radiatively transferred cosmic ultraviolet background due to QSOs dominates the ionization of the absorbers. Extending to redshifts up to 4.4 the additional contribution from galaxies with deduced specific characteristics in the largely unobserved region 1-4 Rydberg contributes increasingly strongly relative to the falling intensity of the QSO background, and includes both the general galaxy background radiation and the inferred local radiation from individual galaxies associated with the absorbers.