The detectability of strong 21-cm forest absorbers in late reionisation models.

A late end to reionisation at redshift z=5.3 is consistent with the observed spatial variations in the Lyα forest transmission and the deficit of Lyα emitting galaxies around extended absorption troughs at z=5.5. In this picture, large islands of neutral hydrogen should persist in the diffuse intergalactic medium (IGM) until z=6. In this talk, I will present state-of- the-art models of the forest of 21-cm absorption lines that will arise from this neutral hydrogen, obtained using high resolution cosmological hydrodynamical simulations coupled with radiative transfer. I include the effects of redshift space distortions and hydrodynamical response to the reionisation on the simulated 21-cm forest spectra. These effects have typically been neglected in the existing 21-cm absorption models. I will show that strong 21-cm lines (>1% absorption) should persist in the spectra of high-redshift radio sources until z=6, for the case of only modest IGM pre-heating with gas kinetic temperatures of T~100K. Forthcoming observations of 21-cm forest absorbers at z=6-9 with SKA1-low or possibly LOFAR should therefore provide an informative lower limit on the still largely unconstrained soft X-ray background at high redshift.