A statistical measurement of the HI spin temperature in DLAs at cosmological distances
2021
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.
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