Discovery of 21-cm absorption in a zabs = 2.289 damped Lyman system towards TXS 0311+430: the first low spin temperature absorber at z > 1

Abstract

We report the detection of H I 21-cm absorption from the z= 2.289 damped Lyman α system (DLA) towards TXS 0311+430 with the Green Bank Telescope. The 21-cm absorption has a velocity spread (between nulls) of ∼110 km s−1 and an integrated optical depth of graphic. We also present new Giant Metrewave Radio Telescope 602-MHz imaging of the radio continuum. TXS 0311+430 is unresolved at this frequency, indicating that the covering factor of the DLA is likely to be high. Combining the integrated optical depth with the DLA H I column density of graphic yields a spin temperature of Ts= (138 ± 36) K, assuming a covering factor of unity. This is the first case of a low spin temperature (<350 K) in a z > 1 DLA and is among the lowest temperatures ever measured in any DLA. Indeed, the Ts measured for this DLA is similar to values measured in the Milky Way and local disc galaxies. We also determine a lower limit (Si/H) ≳ 1/3 solar for the DLA metallicity, amongst the highest abundances measured in DLAs at any redshift. Based on low-redshift correlations, the low Ts, large 21-cm absorption width and high metallicity all suggest that the z∼ 2.289 DLA is likely to arise in a massive, luminous disc galaxy.