Cake Talk by Ivelin Georgiev

The Epoch of Reionisation (EoR) was the period of the Universe during which the neutral hydrogen present in the intergalactic medium (IGM) was ionised by Ultraviolet radiation from sources inside early galaxies. There has been significant progress in modelling cosmic reionisation through numeric and semi-numeric codes. However, the end of reionisation is less studied. In radiative transfer simulations, the opacity of the ionised IGM for ionising photons becomes important during the end of the EoR, and there exists quite a bit of uncertainty about this quantity. While measurements of the Ultraviolet Background (UVB) at z = 6 exist, many simulations struggle to reproduce this number and report higher UVB values.

In this talk, I address how the final stages of reionisation depend on assumptions about the sources, the clumping of the intergalactic medium and the presence of small-scale absorbers (so-called Lyman Limit Systems). In large-scale simulations of the EoR, the latter are often modelled through a parameter to account for the mean free path of ionising photons (MFP) in the IGM. We find that a shorter MFP, more consistent with current measurements from Becker et al. 2021, not only extends reionisation but regulates the UVB and IGM opacity. Moreover, we show how the numerical implementation of MFP impacts the evolution of the UVB.