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 ($\hat{I}{»}_{912}^{\mathrm{mfp}}$) 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 $\hat{I}{»}_{912}^{\mathrm{mfp}}=3.57_{-2.14}^{+3.09}$ 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 $\hat{I}{»}_{912}^{\mathrm{mfp}}$ 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 $\hat{I}{»}_{912}^{\mathrm{mfp}}(z=6)$ points to faint galaxies as the primary drivers of reionizaton. Our preferred model, with $\hat{I}{»}_{912}^{\mathrm{mfp}}(z=6)=6.5$ 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 $\log (f_{\mathrm{esc}}^{\mathrm{eff}}\hat{I}{¾}_{\mathrm{ion}}/[{\mathrm{erg}}^{-1}\mathrm{Hz}])=24.8$ at $z>6$. Recovering $\hat{I}{»}_{912}^{\mathrm{mfp}}(z=6)$ 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.

8 pages, 3 figures