Recombination Clumping Factor of Physically Defined Intergalactic Medium at the Epoch of Reionization

Abstract The recombination clumping factor, C , is a key parameter in modeling cosmic reionization, but its value is sensitive to the definition of the intergalactic medium (IGM). We investigate the clumping factor using the Gamer-2 adaptive mesh refinement cosmological hydrodynamical simulation code. We introduce a new, physically motivated definition of the IGM based on the effective transmission factor of ionizing photons. We perform large-scale simulations with varying intensities of the uniform ultraviolet background, and we find that our physically defined clumping factor is slightly lower than, yet comparable to, the values derived from traditional overdensity thresholds, within a factor of 2. At z = 6, we obtain a clumping factor of C ∼ 3, consistent with previous studies, indicting that the clumping factor is robust to numerical resolution, box size, and the definition of the IGM. Our zoom-in simulations further show that supernova feedback has two competing effects on reionization; it enhances recombination by increasing the density of ionized gas while facilitating ionization by heating gas and reducing the neutral fraction. However, these effects are limited to the scales of ∼100 kpc and do not significantly alter the global clumping factor.