Robust Ly alpha forest constraints on reionization from semi-analytic galaxy formation models

The high-redshift Ly alpha forest provides one of the most sensitive probes of the final stages of reionization, but its interpretation can depend on how the ionizing sources are modelled. We test the robustness of Ly alpha forest-inferred reionization histories using Meraxes, a semi-analytic model of galaxy formation and reionization applied to a cosmological volume of (210h-1 cMpc) cubed with merger trees augmented down to the atomic-cooling threshold. We construct a suite of source models in which the galaxy escape fraction, f esc gal, is parameterized as a function of halo mass, stellar mass, star formation rate, specific star formation rate, or star-forming disc surface density. We also consider models in which accreting blackholes contribute ionizing photons, or dense circumgalactic gas further attenuates the escape of ionizing photons. Each model is calibrated to the observed distribution of Ly alpha effective optical depths from XQR-30+. These prescriptions redistribute the escaping photon budget across halo mass and UV luminosity, introduce scatter in f esc gal at fixed halo mass, and allow modest variation in the early evolution of reionization. Nevertheless, all viable calibrated models recover a late and extended EoR, with the midpoint occurring at z about6.2 --6.9 and the final stages at z about5.36 --5.57. Our results indicate that the late reionization history inferred from Ly alpha forest is robust to physically motivated scatter in the galaxy--halo connection, modest AGN contributions, and phenomenological CGM attenuation.