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-Hi-weighted harmonic mean spin temperature (T-s) at redshifts between z = 0.37 and 1.0. We find that T-s <= 274K with 95percent 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-s,T- MW similar to 300K). This result is consistent with an evolving CNM fraction that mirrors the molecular gas towards the SFR peak at z similar to 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.

Automated summary

This study uses Bayesian statistical modeling to analyze the temperature of Hi gas in galaxies at specific redshifts, indicating that the Hi gas may be colder than the Milky Way interstellar medium. Future surveys are expected to provide more precise constraints on the fraction of cold neutral medium.