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Evolution of the cosmic star formation rate (SFR) and molecular gas massdensity is expected to be matched by a similarly strong evolution of thefraction of atomic hydrogen (HI) in the cold neutral medium (CNM). We useresults from a recent commissioning survey for intervening 21-cm absorbers withthe Australian Square Kilometre Array Pathfinder (ASKAP) to construct aBayesian statistical model of the $N_{\rm HI}$-weighted harmonic mean spintemperature ($T_{\rm s}$) at redshifts between $z = 0.37$ and $1.0$. We findthat $T_{\rm s} \leq 274$K with 95 per cent probability, suggesting that atthese redshifts the typical HI gas in galaxies at equivalent DLA columndensities 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 thatmirrors the molecular gas towards the SFR peak at $z \sim 2$. We expect thatfuture surveys for HI 21-cm absorption with the current SKA pathfindertelescopes will provide constraints on the CNM fraction that are an order ofmagnitude greater than presented here.

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