The detectability of strong 21 centimetre forest absorbers from the diffuse intergalactic medium in late reionisation models

TL;DR

This study predicts the presence of strong 21 cm forest absorption lines in the late reionisation epoch, using simulations to connect their detectability with the IGM temperature and X-ray background, informing future observations.

Contribution

It introduces a hybrid simulation approach combining hydrodynamics and radiative transfer to predict 21 cm forest features in late reionisation models, accounting for redshift distortions and IGM heating uncertainties.

Findings

Strong 21 cm absorption lines should persist until z=6 in low-temperature IGM models.

Null detection at z=6 can constrain high-redshift X-ray background and IGM temperature.

Detection prospects depend on the number of suitable radio sources and instrument sensitivity.

Abstract

A late end to reionisation at redshift $z\simeq 5.3$ is consistent with observed spatial variations in the Ly$\alpha$ forest transmission and the deficit of Ly$\alpha$ emitting galaxies around extended Ly$\alpha$ absorption troughs at $z=5.5$. In this model, large islands of neutral hydrogen should persist in the diffuse intergalactic medium (IGM) until $z\simeq 6$. We use a novel, hybrid approach that combines high resolution cosmological hydrodynamical simulations with radiative transfer to predict the incidence of strong 21 cm forest absorbers with optical depths $\tau_{21}>10^{-2}$ from the diffuse IGM in these late reionisation models. We include the effect of redshift space distortions on the simulated 21 cm forest spectra, and treat the highly uncertain heating of the pre-reionisation IGM by soft X-rays as a free parameter. For a model with only modest IGM pre-heating, such that average gas kinetic temperatures in the diffuse IGM remain below $T_{\rm K}\simeq 10^{2} \rm\, K$, we find that strong 21 cm forest absorption lines should persist until $z=6$. For a sample of $\sim 10$ sufficiently radio loud background sources, a null-detection of 21 cm forest absorbers at $z\simeq 6$ with SKA1-low or possibly LOFAR should provide an informative lower limit on the still largely unconstrained soft X-ray background at high redshift and the temperature of the pre-reionisation IGM.