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

TL;DR

This study places strong lower limits on the H I spin temperature in two high-redshift Damped Lyman-alpha systems, supporting the idea of increasing warm gas fraction with redshift and providing insights into galaxy evolution.

Contribution

The paper provides the first stringent constraints on the spin temperature of Damped Lyman-alpha systems at z > 3 using low-frequency radio observations.

Findings

H I spin temperature > 2000 K for both absorbers

High spin temperatures align with existing temperature trends at high redshift

Temperature-metallicity relation is not due to line-of-sight effects

Abstract

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.