Discovery of 21cm absorption in a $z_{\rm abs} =2.289$ DLA towards TXS 0311+430: The first low spin temperature absorber at z > 1

TL;DR

This paper reports the first low spin temperature 21cm absorption detection in a high-redshift DLA, indicating the presence of a massive, luminous disk galaxy at z > 1 with high metallicity.

Contribution

It presents the discovery of a low spin temperature in a z > 1 DLA, challenging previous assumptions and suggesting the presence of a massive galaxy at high redshift.

Findings

First low spin temperature (< 350 K) in a z > 1 DLA.

DLA metallicity is at least one-third solar.

Indicates the DLA likely arises in a massive, luminous disk galaxy.

Abstract

We report the detection of HI 21 cm absorption from the $z=2.289$ damped Lyman-$\alpha$ system (DLA) towards TXS 0311+430, with the Green Bank Telescope. The 21 cm absorption has a velocity spread (between nulls) of $\sim 110$ km s$^{-1}$ and an integrated optical depth of $\int \tau {\rm d}V = (0.818 \pm 0.085)$ km s$^{-1}$. 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 HI column density of \nhi = $(2 \pm 0.5) \times 10^{20}$ \cm, yields a spin temperature of $T_s = (138 \pm 36)$ 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 ever measured in any DLA. Indeed, the $T_s$ measured for this DLA is similar to values measured in the Milky Way and local disk galaxies. We also determine a lower limit (Si/H) $\gtrsim 1/3$ solar for the DLA metallicity, amongst the highest abundances measured in DLAs at any redshift. Based on low redshift correlations, the low $T_s$, large 21 cm absorption width and high metallicity all suggest that the $z \sim 2.289$ DLA is likely to arise in a massive, luminous disk galaxy.