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

Evolution of the cosmic star formation rate (SFR) and molecular 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 $N_\mathrm{HI}$-weighted harmonic mean spin temperature ($T_\mathrm{s}$) at redshifts between $z = 0.37$ and 1.0. We find that $T_\mathrm{s} \leq 274$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 $(T_\mathrm{s,MW} \sim 300$K). This result is consistent with an evolving CNM fraction that mirrors the molecular gas towards the peak in SFR at $z \sim 2$. We expect that future surveys for HI 21-cm absorption with the current SKA pathfinder telescopes will be able to provide constraints on the CNM fraction that are an order of magnitude greater than presented here.