Hydrodynamic Response of the Intergalactic Medium to Reionization. II. Physical Characteristics and Dynamics of Ionizing Photon Sinks

Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium (IGM) at z = 6. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman-limit opacity by tracking the column-density distributions in our simulations. Within Δt = 10 Myr of I-front passage, we find that self-shielding systems (N _{H I} > 10^17.2 cm^-2) are comprised of two distinct populations: (1) overdensity Δ ~ 50 structures in photoionization equilibrium with the ionizing background, and (2) Δ ≳ 100 density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in Δt ~ 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Δt = 300 Myr, they contribute ≲10% to the opacity in an average 1 Mpc³ patch of the universe. The percentage can be a factor of a few larger in overdense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find M = 10⁴-10⁸ M _⊙ and effective diameters d _eff = 1-20 ckpc h ^-1. These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power.