Finding the First Water in the Universe

Sophie Thorne

doi:10.59973/emjsr.390

Authors

Sophie Thorne School of Mathematics and Physics, University of Portsmouth, PO1 3QL Portsmouth, UK

DOI:

https://doi.org/10.59973/emjsr.390

Keywords:

Primordial Water, Cosmic Dawn, Radiative Transfer

Abstract

This article investigates the observational detectability of water formed in primordial supernova remnants during the epoch of Cosmic Dawn (z ∼ 15–20). Recent cosmological simulations have shown that significant water abundances can arise in dense, metal-enriched cloud cores following Population III supernovae. However, the observational feasibility of detecting such primordial water has remained unclear. To address this, radiative transfer modelling is performed using RADMC-3D, adopting physical conditions derived from high-resolution Enzo simulations. Adaptive mesh refinement simulation data is resampled onto a uniform grid and used to compute molecular emission from a large set of rotational water transitions under the assumption of local thermodynamic equilibrium. A high-throughput computational pipeline is developed to efficiently evaluate line emission across many transitions, enabling the construction of the full water emission spectrum.
The resulting intrinsic flux densities are redshifted to z = 22.2 and scaled using luminosity-distance relations to obtain observable flux densities. These are compared directly with the sensitivity limits of the Atacama Large Millimeter / Submillimeter Array (ALMA), with the emission grouped into relevant observational bands. Background flux calculations are performed following the general formalism presented by for cosmological radiation backgrounds. Instead of adopting a halo-mass-dependent luminosity relation, the luminosity derived from radiative-transfer modelling yields a characteristic flux density for each emitting cloud in each ALMA observing band.
The results show that the strongest signals from individual clouds lie three to four orders of magnitude below the ALMA detection thresholds, making them observationally inaccessible. However, flux densities in the background flux are increased by five orders of magnitude due to several objects falling in a single ALMA beam. Flux densities of the background water-line haze are therefore potentially detectable with current instrumentation within reasonable time frames.

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