Abstract
We use a suite of cosmological hydrodynamic simulations including a
self-consistent treatment for inhomogeneous reionisation to study the impact of
galactic outflows and photoionisation heating on the volume-averaged
recombination rate of the intergalactic medium (IGM). By incorporating an
evolving ionising escape fraction and a treatment for self-shielding within
Lyman limit systems, we have run the first simulations of "photon-starved"
reionisation scenarios that simultaneously reproduce observations of the
abundance of galaxies, the optical depth to electron scattering of cosmic
microwave background photons \tau, and the effective optical depth to
Lymanabsorption at z=5. We confirm that an ionising background reduces
the clumping factor C by more than 50% by smoothing moderately-overdense
(\Delta=1--100) regions. Meanwhile, outflows increase clumping only modestly.
The clumping factor of ionised gas is much lower than the overall baryonic
clumping factor because the most overdense gas is self-shielded.
Photoionisation heating further suppresses recombinations if reionisation 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 ionisation-corrected clumping factors into an
analytical reionisation model reproduces the numerical simulation's to
within 10%. Finally, we explore how many ionising photons are absorbed during
the process of heating filaments by considering the overall photon cost of
reionisation in analytical models that assume that the IGM is heated at
different redshifts. For reionisation redshifts of 9--10, cold filaments boost
the reionisation photon budget by ~1 photon per hydrogen atom.
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