Abstract
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 (H i) 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 NH i-weighted harmonic mean spin temperature (Ts) at redshifts between z = 0.37 and 1.0. We find that Ts ≤ 274 K with 95 per cent probability, suggesting that at these redshifts the typical H i gas in galaxies at equivalent DLA column densities may be colder than the Milky Way interstellar medium (Ts, MW ∼ 300 K). This result is consistent with an evolving CNM fraction that mirrors the molecular gas towards the SFR peak at z ∼ 2. We expect that future surveys for H i 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.
Highlights
The coldest (Tk < 100 K) interstellar gas has a fundamental role in forming stars and fuelling galaxy evolution throughout cosmic history
Stars form in the dusty molecular gas of the interstellar medium (ISM) and so it is believed that this phase is most important in fuelling evolution of the star formation rate (SFR) density throughout cosmic history
The denser cold neutral medium (CNM; Tk ∼ 100 K) and more diffuse warm neutral medium (WNM; Tk ∼ 10 000 K), co-exist in a pressure equilibrium that is determined by heating and cooling processes that are dependent on the star formation rate, dust abundance and gas-phase metallicity (Wolfire et al 2003)
Summary
The coldest (Tk < 100 K) interstellar gas has a fundamental role in forming stars and fuelling galaxy evolution throughout cosmic history. The denser cold neutral medium (CNM; Tk ∼ 100 K) and more diffuse warm neutral medium (WNM; Tk ∼ 10 000 K), co-exist in a pressure equilibrium that is determined by heating and cooling processes that are dependent on the star formation rate, dust abundance and gas-phase metallicity (Wolfire et al 2003). Further dynamical processes, such as turbulence and supernova shocks, are thought to generate an unstable third phase at intermediate temperatures If the presence of CNM is a pre-requisite for molecular cloud and star formation (e.g. Krumholz et al 2009), a strong redshift evolution would be expected for the CNM fraction
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