The detectability of strong 21 centimetre forest absorbers from the diffuse intergalactic medium in late reionisation models
Tomáš Šoltinskí, James S. Bolton, N. A. Hatch, Martin G. Haehnelt, Laura C. Keating, Girish Kulkarni, Ewald Puchwein, Jonathan Chardin, Dominique Aubert
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.