By depleting the ionizing photon budget available to expand cosmic HII regions , recombining systems ( or Lyman limit systems ) can have a large impact during ( and following ) cosmic reionization .
Unfortunately , directly resolving such structures in large-scale reionization simulations is computationally impractical .
Instead , here we implement a sub-grid prescription for tracking inhomogeneous recombinations in the intergalactic medium .
Building on previous work parameterizing photo-heating feedback on star-formation , we present large-scale , semi-numeric reionization simulations which self-consistently track the local ( sub-grid ) evolution of both sources and sinks of ionizing photons .
Our simple , single-parameter model naturally results in both an extended reionization and a modest , slowly-evolving emissivity , consistent with observations .
Recombinations are instrumental in slowing the growth of large HII regions , and damping the rapid rise of the ionizing background in the late stages of ( and following ) reionization .
As a result , typical HII regions are smaller by factors of \sim 2–3 throughout reionization .
The large-scale ( k \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } } 0.2 Mpc ^ { -1 } ) ionization power spectrum is suppressed by factors of \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ > $ } } 2 –3 in the second half of reionization .
Therefore properly modeling recombinations is important in interpreting virtually all reionization observables , including upcoming interferometry with the redshifted 21cm line .
Consistent with previous works , we find the clumping factor of ionized gas to be C _ { HII } \sim 4 at the end of reionization .