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Coupled radiative-hydrodynamics simulations of the epoch of reionization aimto reproduce the propagation of ionization fronts during the transition beforethe overlap of HII regions. Many of these simulations use moment-based methodsto track radiative transfer processes using explicit solvers and are thereforesubject to strict stability conditions regarding the speed of light, whichimplies a great computational cost. It can be reduced by assuming a reducedspeed of light, and this approximation is now widely used to producelarge-scale simulations of reionization. We introduce a new method forestimating and comparing the ionization front speeds based on maps of thereionization redshifts. We applied it to a set of cosmological simulations ofthe reionization using a set of reduced speeds of light, and measured theevolution of the ionization front speeds during the reionization process. Wefind that ionization fronts progress via a two-stage process, the first stageat low velocity as the fronts emerge from high density regions and a secondlater stage just before the overlap, during which front speeds increase closeto the speed of light. Using a set of small 8Mpc/h^3 simulations, we find thata minimal velocity of 0.3c is able to model these two stages in this specificcontext without significant impact. Values as low as 0.05c can model the firstlow velocity stage, but limit the acceleration at later times. Lower valuesmodify the distribution of front speeds at all times. Using larger 64Mpc/h^3volumes that better account for distant sources, we find that reduced speed oflight has a greater impact on reionization times and front speeds in underdenseregions that are reionized at late times. The same quantities measured in denseregions with slow fronts are less sensitive to c values.

Impact of the reduced speed of light approximation on ionization front velocities in cosmological simulations of the epoch of reionization