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

Physical properties of Damped Lyman-alpha absorbers and their evolution are closely related to galaxy formation and evolution theories, and have important cosmological implications. H I 21 cm absorption study is one useful way of measuring the temperature of these systems. In this work, very strong constraints on the temperature of two Damped Lyman-alpha absorbers at z > 3 are derived from low radio frequency observations. The H I spin temperature is found to be greater than 2000 K for both the absorbers. The high spin temperature of these high-redshift systems is in agreement with the trend found in a compilation of temperatures for other Damped Lyman-alpha absorbers. We also argue that the temperature - metallicity relation, reported earlier in the literature, is unlikely to be a spurious line of sight effect, and that the redshift evolution of the spin temperature does not arises due to a selection effect. All of these are consistent with a redshift evolution of the warm gas fraction in Damped Lyman-alpha systems.