Hydrodynamic Response of the Intergalactic Medium to Reionization II:
  Physical Characteristics and Dynamics of Ionizing Photon Sinks
  Becker et al. 2021 measured the mean free path of Lyman limit photons in the
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 ($N_{\rm HI} > 10^{17.2}$ cm$^{-2}$) are comprised of
two distinct populations: (1) over-density $\Delta \sim 50$ structures in
photo-ionization equilibrium with the ionizing background; (2) $\Delta \gtrsim
100$ 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 $\Delta t \sim 100$ Myr as structures are flattened by pressure
smoothing and photoevaporation. By $\Delta t = 300$ Myr, they contribute
$\lesssim 10 \%$ to the opacity in an average 1 Mpc$^3$ patch of the Universe.
The percentage can be a factor of a few larger in over-dense 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$_\odot$ and effective diameters $d_{\rm eff} = 1 - 20$
ckpc$/h$. These scales increase as the gas relaxes. The picture herein
presented may be different in dark matter models with suppressed small-scale
power.
