Stringent constraints on the hi spin temperature in two z > 3 damped Lyman α systems from redshifted 21cm absorption studies.

Physical properties of damped Lyman α (DLA) 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 DLA 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 DLA 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 arise due to a selection effect. All of these are consistent with a redshift evolution of the warm gas fraction in DLA systems.