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Published online by Cambridge University Press: 21 April 2007
A complementary method to the emission selection of high-redshiftgalaxies consists in the observation of absorbers along the line ofsight toward a background quasar. This selection technique has aconstant sensitivity at all redshifts up to z = 6 (i.e. no redshiftdesert) and allow to select all types of galaxies regardless of theirluminosity or star formation rate. The highest column densityabsorbers, the Damped Lyman-α (DLAs) systems, in particular,can be used to determine the cosmic evolution of HI gas in theUniverse, ΩHI, and the global metallicity in the gasphase. Since stars are known to form from HI gas, ΩHIprovides an indirect tracer of the history of starformation. Recent results from several parallel VLT programmes aimingat determining the cosmological evolution of the metallicity in theneutral gas phase are presented.