A statistical measurement of the HI spin temperature in DLAs at cosmological distances

TL;DR

This study uses ASKAP data to statistically estimate the HI spin temperature in DLAs at redshifts 0.37 to 1.0, indicating colder gas than in the Milky Way and supporting an evolving cold neutral medium fraction linked to star formation history.

Contribution

It introduces a Bayesian model to measure the HI spin temperature at intermediate redshifts using 21-cm absorption data, providing new constraints on the cold neutral medium in distant galaxies.

Findings

HI spin temperature is likely below 274K at z=0.37-1.0

HI gas in these galaxies may be colder than in the Milky Way

Results support an evolving cold neutral medium fraction with cosmic star formation

Abstract

Evolution of the cosmic star formation rate (SFR) and molecular gas mass density is expected to be matched by a similarly strong evolution of the fraction of atomic hydrogen (HI) in the cold neutral medium (CNM). We use results from a recent commissioning survey for intervening 21-cm absorbers with the Australian Square Kilometre Array Pathfinder (ASKAP) to construct a Bayesian statistical model of the $N_{\rm HI}$-weighted harmonic mean spin temperature ($T_{\rm s}$) at redshifts between $z = 0.37$ and $1.0$. We find that $T_{\rm s} \leq 274$K with 95 per cent probability, suggesting that at these redshifts the typical HI gas in galaxies at equivalent DLA column densities may be colder than the Milky Way interstellar medium ($T_{\rm s, MW} \sim 300$K). This result is consistent with an evolving CNM fraction that mirrors the molecular gas towards the SFR peak at $z \sim 2$. We expect that future surveys for HI 21-cm absorption with the current SKA pathfinder telescopes will provide constraints on the CNM fraction that are an order of magnitude greater than presented here.