Computationally efficient reionization in a large hydrodynamic galaxy formation simulation

Accuracy in the topology and statistics of a simulated epoch of reionization (EoR) are vital to draw connections between observations and physical processes. While full radiative transfer models produce the most accurate reionization models, they are highly computationally expensive, and are infeasible for the largest cosmological simulations. Instead, large simulations often include EoR models that are pre-computed via the initial density field, or post-processed where feedback effects are ignored. We introduce ASTRID-ES, a resimulation of the ASTRID epoch of reionization 20 > z > 5.5 which includes an on-the-fly excursion-set reionization algorithm. ASTRID-ES produces more accurate reionization histories without significantly impacting the computational time. This model directly utilizes the star particles produced in the simulation to calculate the EoR history and includes an ultraviolet (UV) background which heats the gas particles after their reionization. We contrast the reionization topology and statistics in ASTRID-ES with the previously employed parametric reionization model, finding that in ASTRID-ES, ionized regions are more correlated with galaxies, and the 21cm power spectrum shows an increase in large-scale power. We calculate the relation between the size of H II regions and the UV luminosity of the brightest galaxy within them. Prior to the overlap phase, we find a power-law fit of log(R) = -0.314M(UV) - 2.550log(1 + z) + 7.408 with a standard deviation sigma(R) < 0.15 dex across all mass bins. We also examine the properties of haloes throughout reionization, finding that while the properties of haloes in the simulation are correlated with the redshift of reionization, they are not greatly affected by reionization itself.