Simulating ionization feedback from young massive stars: impact of numerical resolution

Modelling galaxy formation in hydrodynamic simulations has increasingly adopted various radiative transfer methods to account for photoionization feedback from young massive stars. However, the evolution of H II regions around stars begins in dense star-forming clouds and spans large dynamical ranges in both space and time, posing severe challenges for numerical simulations in terms of both spatial and temporal resolution that depends strongly on gas density (proportional to n(-1)). In this work, we perform a series of idealized H II region simulations using the moving-mesh radiation-hydrodynamic code AREPO-RT to study the effects of numerical resolution. The simulated results match the analytical solutions and the ionization feedback converges only if the Str & ouml;mgren sphere is resolved by at least 10-100 resolution elements and the size of each time integration step is smaller than 0.1 times the recombination time-scale. Insufficient spatial resolution leads to reduced ionization fraction but enhanced ionized gas mass and momentum feedback from the H II regions, as well as degrading the multiphase interstellar medium into a diffuse, partially ionized, warm (similar to 8000 K) gas. On the other hand, insufficient temporal resolution strongly suppresses the effects of ionizing feedback. This is because longer time-steps are not able to resolve the rapid variation of the thermochemistry properties of the gas cells around massive stars, especially when the photon injection and thermochemistry are performed with different cadences. Finally, we provide novel numerical implementations to overcome the above issues when strict resolution requirements are not achievable in practice.