A search for HI 21cm absorption towards a radio-selected quasar sample II: a new low spin temperature DLA at high redshift

TL;DR

This study searches for HI 21cm absorption in high-redshift DLAs, discovering two with low spin temperatures similar to local galaxies, indicating they may be massive, metal-rich galaxies, and provides insights into the nature of high-z DLAs.

Contribution

First detection of low spin temperature DLAs at z > 1, linking their properties to massive, high-metallicity galaxy hosts, and expanding understanding of high-redshift galaxy environments.

Findings

Detected HI 21cm absorption in two high-z DLAs with low spin temperatures.

Established lower limits on spin temperatures for non-detections, consistent with high-z DLA population.

Suggested that low spin temperature DLAs are likely associated with massive, metal-rich galaxies.

Abstract

We report results from a deep search for redshifted HI 21cm absorption from eight damped Lyman-$\alpha$ absorbers (DLAs) detected in our earlier optical survey for DLAs towards radio-loud quasars. HI 21cm absorption was detected from the $z = 2.192$ DLA towards TXS2039+187, only the sixth case of such a detection at $z > 2$, while upper limits on the HI 21cm optical depth were obtained in six other DLAs at $z > 2$. Our detection of HI 21cm absorption in the eighth system, at $z = 2.289$ towards TXS0311+430, has been reported earlier. We also present high spatial resolution images of the background quasars at frequencies close to the redshifted HI 21cm line frequency, allowing us to estimate the covering factor of each DLA, and then determine its spin temperature $T_s$. For three non-detections of HI 21cm absorption, we obtain strong lower limits on the spin temperature, $T_s \gtrsim 790$ K, similar to the bulk of the high-$z$ DLA population; three other DLAs yield weak lower limits, $T_s > 140-400$ K. However, for the two DLAs with detections of HI 21cm absorption, the derived spin temperatures are both low $T_s = (160 \pm 35) \times (f/0.35)$ K for the $z = 2.192$ DLA towards TXS2039+187 and $T_s = (72 \pm 18) \times (f/0.52)$ K for the $z = 2.289$ DLA towards TXS0311+430. These are the first two DLAs at $z > 1$ with $T_s$ values comparable to those obtained in local spiral galaxies. Based on the observed correlations between spin temperature and metallicity and velocity spread and metallicity in DLAs, we suggest that the hosts of the two absorbers are likely to be massive, high-metallicity galaxies.