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arXiv · 2102.09927

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

James R. Allison
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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.

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James R. Allison. 2021-03-15. A statistical measurement of the HI spin temperature in DLAs at cosmological distances. https://doi.org/10.1093/mnras%2Fstab518

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