Late-end reionization with ATON-HE: towards constraints from Ly α emitters observed with JWST

We present a new suite of late-end reionization simulations performed with ATON-HE, a revised version of the GPU-based radiative transfer code ATON that includes helium. The simulations are able to reproduce the Ly α flux distribution of the E-XQR-30 sample of QSO absorption spectra at 5< z < 6.2, and show that a large variety of reionization models are consistent with these data. We explore a range of variations in source models and in the early-stage evolution of reionization. Our fiducial reionization history has a mid-point of reionization at z = 6.5, but we also explore an ‘Early’ reionization history with a mid-point at z = 7.5 and an ‘Extremely Early’ reionization history with a mid-point at z = 9.5. Haloes massive enough to host observed Ly α emitters are highly biased. The fraction of such haloes embedded in ionized bubbles that are large enough to allow high Ly α transmission
becomes close to unity much before the volume filling factor of ionized regions. For our fiducial reionization history this happens at z = 8, probably too late to be consistent with the detection by JWST of abundant Ly α emission out to z = 11. A reionization history in our ‘Early’ model or perhaps even our ‘Extremely Early’ model may be required, suggesting a Thomson scattering optical depth in tension with that reported by Planck, but consistent with recent suggestions of a significantly higher value.