Gas clumping in self-consistent reionization models

We use a suite of cosmological hydrodynamic simulations including a self-consistent treatment for inhomogeneous reionization to study the impact of galactic outflows and photoionization heating on the volume-averaged recombination rate of the intergalactic medium (IGM). By incorporating an evolving ionizing escape fraction and a treatment for self-shielding within Lyman limit systems, we have run the first simulations of photon-starved reionization scenarios that simultaneously reproduce observations of the abundance of galaxies, the optical depth to electron scattering of cosmic microwave background photons tes and the effective optical depth to Lya absorption at z = 5. We confirm that an ionizing background reduces the clumping factor C by more than 50?per?cent by smoothing moderately overdense (? = 1100) regions. Meanwhile, outflows increase clumping only modestly. The clumping factor of ionized gas is much lower than the overall baryonic clumping factor because the most overdense gas is self-shielded. Photoionization heating further suppresses recombinations if reionization heats gas above the canonical 10?000?K. Accounting for both effects within our most realistic simulation, C rises from <1 at z > 10 to 3.3 at z = 6. We show that incorporating temperature- and ionization-corrected clumping factors into an analytical reionization model reproduces the numerical simulation's tes to within 10?per?cent. Finally, we explore how many ionizing photons are absorbed during the process of heating filaments by considering the overall photon cost of reionization in analytical models that assume that the IGM is heated at different redshifts. For reionization redshifts of 910, cold filaments boost the reionization photon budget by similar to 1 photon per hydrogen atom.