Kavli Affiliate: Mark Vogelsberger
| Summary:
Recombinations during the Epoch of Reionization are intrinsically inhomogeneous, with different regions of the intergalactic medium contributing unevenly depending on their density, temperature, ionization state, and spatial patchiness. We combine the high- and medium-resolution 95.5 cMpc Thesan-1 andh Thesan-2 runs with the significantly larger 500 cMpc Lumina simulation to measure clumping factors and recombination rates consistently across different resolutions and box sizes. We consider the standard ionized hydrogen clumping factor, $C_rm HII equiv langle n_rm HII^2rangle/langle n_rm HIIrangle^2$, and a recombination-weighted clumping factor, $C_rm rec$. Despite differences in resolution, volume, and reionization history, the simulations show an approximately universal clumping evolution at the 10-20% level when parametrized by the global ionized fraction $x_rm HII$ rather than by redshift. Across all simulations, $C_rm rec$ remains systematically below $C_rm HII$, with the discrepancy increasing toward lower redshift as photoheating suppresses recombinations. In lumina, the density-only prescription overpredicts the instantaneous recombination rate by factors of 1.29 at $zapprox8$ and 1.84 at $zapprox5$, and the cumulative recombination count by a factor of 1.45 by $zapprox5$. Mapping the recombination budget in the joint overdensity-temperature plane reveals that the dominant recombination ridges closely follow simple analytic thermal equilibrium bands. Finally, we introduce a phase-space recombination integral and define a phase-space clumping factor, $C_rm ps(Δ,T)$, which isolates the intrinsic recombination enhancement associated with ionization structure and thermal state at fixed overdensity and temperature.
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