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

Abstract

Abstract A late end to reionisation at redshift z ≃ 5.3 is consistent with observed spatial variations in the Lyα forest transmission and the deficit of Lyα emitting galaxies around extended Lyα absorption troughs at z = 5.5. In this model, large islands of neutral hydrogen should persist in the diffuse intergalactic medium (IGM) until z ≃ 6. We use a novel, hybrid approach that combines high resolution cosmological hydrodynamical simulations with radiative transfer to predict the incidence of strong $21\rm , cm$ forest absorbers with optical depths τ21 > 10−2 from the diffuse IGM in these late reionisation models. We include the effect of redshift space distortions on the simulated $21\rm , cm$ 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 $21\rm , cm$ forest absorption lines should persist until z = 6. For a sample of ∼10 sufficiently radio loud background sources, a null-detection of $21\rm , cm$ forest absorbers at z ≃ 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.