Late reionization and the nature of ionizing photon sinks

The question of what constitutes the sinks of ionizing photons during reionization has been spotlighted by the recent mean free path measurements of Becker et al. 2021. Taken together with previous measurements at z ≲ 5, their short value of 0.75^+0.65_-0.45 pMpc at z = 6 suggests a factor of ~10 evolution over a time span of ~200 Myr. I will discuss theoretical insights into the physical characteristics and dynamics of the sinks, gleaned from radiative hydrodynamics simulations. I will also discuss the challenges that quasar absorption spectra measurements pose for all existing models of reionization.