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

Abstract

A late end to reionization at redshift z ≈ 5.3 is consistent with observed spatial variations in the Ly α forest transmission andτthe deficit of Ly α emitting galaxies around extended Ly α absorption troughs at z = 5.5. In this model, large islands of neutralτhydrogen should persist in the diffuse intergalactic medium (IGM) until z ≈ 6. We use a novel, hybrid approach that combinesτhigh-resolution cosmological hydrodynamical simulations with radiative transfer to predict the incidence of strong 21-cm forestτabsorbers with optical depths τ 21 > 10−2 from the diffuse IGMin these late reionization models.We include the effect of redshiftτspace distortions on the simulated 21-cm forest spectra, and treat the highly uncertain heating of the pre-reionization IGM byτsoft X-rays as a free parameter. For a model with only modest IGM pre-heating, such that average gas kinetic temperatures inτthe diffuse IGM remain below TK ≈ 102 K, we find that strong 21-cm forest absorption lines should persist until z = 6. For aτsample of ∼10 sufficiently radio-loud background sources, a null-detection of 21-cm forest absorbers at z ≈ 6 with SKA1-lowτor possibly LOFAR should provide an informative lower limit on the still largely unconstrained soft X-ray background at highτredshift and the temperature of the pre-reionization IGM.