A Short Mean Free Path at z=6 Favors Late and Rapid Reionization by Faint Galaxies

Recent measurements of the ionizing photon mean free path (lambda(mfp)(912)) at 5 < z < 6 suggest that the IGM was rapidly evolving at those times. We use radiative transfer simulations to explore the implications for reionization, with a focus on the short value of lambda(mfp)(912) = 3.57(-2.14)(+3.09) h(-1) at z = 6. We introduce a model for subresolution ionizing photon sinks based on radiative hydrodynamics simulations of small-scale IGM clumping. We argue that the rapid evolution in lambda(mfp)(912) z = 5 - 6, together with constraints on the metagalactic ionizing background, favors a late reionization process in which the neutral fraction evolved rapidly in the latter half. We also argue that the short lambda(mfp)(912)(z = 6) points to faint galaxies as the primary drivers of reionizaton. Our preferred model, with lambda(mfp)(912)(z = 6) = 6.5 h(-1), has a midpoint of z = 7.1 and ends at z = 5.1. It requires three ionizing photons per H atom to complete reionization and a LyC photon production efficiency of log(f(esc)(eff)xi(ion)/[erg(-1)Hz])=24.8 z > 6. Recovering lambda(mfp)(912) (z = 6) as low as the measured central value may require an increase in IGM clumpiness beyond predictions from simulations, with a commensurate increase in the photon budget.