A short mean free path at z=6 favors late and rapid reionization by faint galaxies

 

arXiv:2105.10511  [pdf, other] 

 

astro-ph.CO

A short mean free path at z=6 favors late and rapid reionization by faint galaxies

Authors: Christopher Cain, Anson D'Aloisio, Nakul Gangolli, George D. Becker

Abstract: Recent measurements of the ionizing photon mean free path (λmfp912) 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 λmfp912=3.57+3.09−2.14 cMpc/h at z=6. We introduce a model for sub-resolution ionizing photon sinks based on radiative hydrodynamics simulations of small-scale IGM clumping. We argue that the rapid evolution in λmfp912 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 λmfp912(z=6) points to faint galaxies as the primary drivers of reionizaton. Our preferred model, with λmfp912(z=6)=6.5 Mpc/h, has a midpoint of z=7.1 and ends at z=5.1. It requires 3 ionizing photons per H atom to complete reionization and a LyC photon production efficiency of log(feffescξion/[erg−1Hz])=24.8 at z>6. Recovering λmfp912(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.