2013MNRAS.436L..94R

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. Hi 21cm 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 Hi spin temperature is found to be greater than 2000K 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.