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