The detectability of strong 21-cm forest absorbers from the diffuse intergalactic medium in late reionization models

A late end to reionization at redshift \(z\simeq5.3\) 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 \(z\simeq6\). 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 reionization 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-reionization 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 \(T_{\rm K}\simeq10^2\,\rm K\), we find that strong 21-cm forest absorption lines should persist until \(z=6\). For a sample of \(\sim10\) sufficiently radio-loud background sources, a null-detection of 21-cm forest absorbers at \(z\simeq6\) 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-reionization IGM.