Introducing the THESAN project: radiation-magnetohydrodynamic simulations of the epoch of reionization

We introduce the THESAN project, a suite of large volume (L = 95.5 cMpc) radiation-magnetohydrodynamic simulations that simultaneously model the large-scale statistical properties of the inter-galactic medium (IGM) during reionization and the resolved characteristics of the galaxies responsible for it. The mass and spatial resolution of the flagship simulations enables predictions down to the atomic cooling limit. The simulations use an efficient radiation hydrodynamics solver (AREPO-RT) that precisely captures the interaction between ionizing photons and gas, coupled to well-tested galaxy formation (IllustrisTNG) and dust models to accurately predict the properties of galaxies. Through a complementary set of medium resolution simulations we investigate the changes to reionization introduced by different assumptions for ionizing escape fractions, varying DM models and numerical convergence. The simulations are calibrated to produce realistic reionization histories that match the observed evolution of the global neutral hydrogen fraction and electron scattering optical depth to reionization. They also match a wealth of high redshift observationally inferred data, including the stellar-to-halo-mass relation, stellar mass functions, star formation rate density and the mass-metallicity relation despite the galaxy formation model being mainly calibrated at z=0. We demonstrate that different reionization models give rise to varied bubble size distributions that imprint unique signatures on the 21cm emission, especially on the slope of the power spectrum at large spatial scales. This enables current and upcoming 21cm experiments to accurately characterize the sources that dominate the ionizing photon budget.