A search for H i 21 cm absorption towards a radio-selected quasar sample – II. A new low spin temperature DLA at high redshift

Abstract

We report results from a deep search for redshifted H I 21 cm absorption from eight damped Lyman α absorbers (DLAs) detected in our earlier optical survey for DLAs towards radio-loud quasars. H I 21 cm absorption was detected from the z = 2.192 DLA towards TXS 2039+187, only the sixth case of such a detection at z > 2, while upper limits on the H I 21 cm optical depth were obtained in six other DLAs at z > 2. Our detection of H I 21 cm absorption in the eighth system, at z = 2.289 towards TXS 0311+430, has been reported earlier. We also present high spatial resolution images of the background quasars at frequencies close to the redshifted H I 21 cm line frequency, allowing us to estimate the covering factor of each DLA, and then determine its spin temperature Ts. For three non-detections of H I 21 cm absorption, we obtain strong lower limits on the spin temperature, Ts ≳ 790 K, similar to the bulk of the high-z DLA population; three other DLAs yield weak lower limits, Ts > 140–400 K. However, for the two DLAs with detections of H I 21 cm absorption, the derived spin temperatures are both low, Ts = (160 ± 35) × (f/0.35) K for the z = 2.192 DLA towards TXS 2039+187 and Ts = (72 ± 18) × (f/0.52) K for the z = 2.289 DLA towards TXS 0311+430. These are the first two DLAs at z > 1 with Ts 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.