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Abstract High-redshift quasi-stellar object (QSO) spectra show large spatial fluctuations in the Ly α opacity of the intergalactic medium on surprisingly large scales at z ≳ 5. 5. We present a radiative transfer simulation of cosmic reionization driven by galaxies that reproduces this large scatter and the rapid evolution of the Ly α opacity distribution at 5 z 6. The simulation also reproduces the low Thomson scattering optical depth reported by the latest cosmic microwave background (CMB) measurement and is consistent with the observed short near-zones and strong red damping wings in the highest redshift QSOs. It also matches the rapid disappearance of observed Ly α emission by galaxies at z ≳ 6. Reionization is complete at z = 5. 3 in our model, and 50 per cent of the volume of the Universe is ionized at z = 7. Agreement with the Ly α forest data in such a late reionization model requires a rapid evolution of the ionizing emissivity of galaxies that peaks at z ∼ 6. 8. The late end of reionization results in a large scatter in the photoionization rate and the neutral hydrogen fraction at redshifts as low as z ≲ 5. 5 with large residual neutral ‘islands’ that can produce very long Gunn–Peterson troughs resembling those seen in the data.
Kulkarni et al. (2019) studied this question.