New searches for HI 21-cm in damped Lyman-alpha absorption systems

TL;DR

This study conducted three searches for HI 21-cm absorption in twelve DLAs and sub-DLAs across redshifts 0.86 to 3.37, analyzing factors affecting detection rates and the properties of absorbers.

Contribution

It provides new observational data and insights into the relationship between 21-cm line strength, neutral hydrogen column density, and absorber geometry at high redshift.

Findings

No absorption detected in five systems with sufficient sensitivity.

Evidence of a correlation between 21-cm line strength and neutral hydrogen column density.

Larger absorbers are more likely to show 21-cm absorption, especially if not in spiral galaxies.

Abstract

We present the results of three separate searches for HI 21-cm absorption in a total of twelve damped Lyman-alpha absorption systems (DLAs) and sub-DLAs over the redshift range z = 0.86-3.37. We find no absorption in the five systems for which we obtain reasonable sensitivities and add the results to those of other recent surveys in order to investigate factors which could have an effect on the detection rate: We provide evidence that the mix of spin temperature/covering factor ratios seen at low redshift may also exist at high redshift, with a correlation between the 21-cm line strength and the total neutral hydrogen column density, indicating a roughly constant spin temperature/covering factor ratio for all of the DLAs searched. Also, by considering the geometry of a flat expanding Universe together with the projected sizes of the background radio emission regions, we find, for the detections, that the 21-cm line strength is correlated with the size of the absorber. For the non-detections it is apparent that larger absorbers (covering factors) are required in order to exhibit 21-cm absorption, particularly if these DLAs do not arise in spiral galaxies. We also suggest that the recent z = 2.3 detection towards TXS 0311+430 arises in a spiral galaxy, but on the basis of a large absorption cross-section and high metallicity, rather than a low spin temperature