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Published online by Cambridge University Press: 07 August 2017
The properties of the QSO absorbing systems have been established by several large scale surveys. Observational results concerning the metal content, the distribution of the systems as a function of the redshift, and clustering, lead to two populations of intervening objects: (a) intergalactic clouds (Ly α systems), and (b) galaxies (heavy-element systems). More recently, observations have shown an analogy between QSO absorbers and gas-rich dwarf galaxies (York et al. 1990).