Modelling recombinations during cosmological reionization

An ionization front expanding into a neutral medium can be slowed down significantly by recombinations. In cosmological numerical simulations the recombination rate is often computed using a 'clumping factor' that takes into account that not all scales in the simulated density field are resolved. Here we demonstrate that using a single value of the clumping factor significantly overestimates the recombination rate, and how a local estimate of the clumping factor is both easy to compute, and gives significantly better numerical convergence. We argue that this lower value of the recombination rate is more relevant during the reionization process and hence that the importance of recombinations during reionization has been overestimated.