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

Abstract

Recent measurements of the ionizing photon mean free path ( λ 912 mfp ) 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 λ 912 mfp = 3.57 − 2.14 + 3.09 cMpc 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 λ 912 mfp at 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 λ 912 mfp ( z = 6 ) points to faint galaxies as the primary drivers of reionizaton. Our preferred model, with λ 912 mfp ( z = 6 ) = 6.5 Mpc 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 ξ ion / [ erg − 1 Hz ] ) = 24.8 at z > 6. Recovering λ 912 mfp ( 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.