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

Journal article

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.

Authors: Allison, JR

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: Monthly Notices of the Royal Astronomical Society
• Volume: 503
• Issue: 1
• Pages: 985-996
• Publication date: 2021-02-24
• Acceptance date: 2021-02-19
• DOI: 10.1093/mnras/stab518
• EISSN: 1365-2966
• ISSN: 0035-8711
• Language: English
• Keywords: quasars: absorption lines; galaxies: ISM; galaxies: evolution; FFR; radio lines: galaxies; methods: statistical