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 Delta t = 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 Delta similar to 50 structures in photoionization equilibrium with the ionizing background, and (2) Delta greater than or similar to 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 Delta t similar to 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Delta t = 300 Myr, they contribute less than or similar to 10% to the opacity in an average 1 Mpc(3) 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 = 10(4)-10(8) M (circle dot) 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.

Automated summary

Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium at z = 6, indicating significant role of absorptions in reionization. They studied physical properties of ionizing photon sinks in the wake of ionization fronts using radiative hydrodynamic simulations. The contributions of self-shielding systems to the opacity decrease over time as the intergalactic medium evolves.