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Physical properties of Damped Lyman-alpha absorbers and their evolution areclosely related to galaxy formation and evolution theories, and have importantcosmological implications. H I 21 cm absorption study is one useful way ofmeasuring the temperature of these systems. In this work, very strongconstraints on the temperature of two Damped Lyman-alpha absorbers at z > 3 arederived from low radio frequency observations. The H I spin temperature isfound to be greater than 2000 K for both the absorbers. The high spintemperature of these high-redshift systems is in agreement with the trend foundin a compilation of temperatures for other Damped Lyman-alpha absorbers. Wealso argue that the temperature - metallicity relation, reported earlier in theliterature, is unlikely to be a spurious line of sight effect, and that theredshift evolution of the spin temperature does not arises due to a selectioneffect. All of these are consistent with a redshift evolution of the warm gasfraction in Damped Lyman-alpha systems.

Stringent constraints on the H I spin temperature in two z > 3 Damped Lyman-alpha systems from redshifted 21 cm absorption studies