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

Abstract

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 $\hat{I}”t=10$ Myr of I-front passage, we find that self-shielding systems ($N_{\mathrm{HI}}>10^{17.2}$ cm${}^{-2}$) are comprised of two distinct populations: (1) over-density $\hat{I}”\sim 50$ structures in photo-ionization equilibrium with the ionizing background; (2) $\hat{I}”\gtrsim 100$ 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 $\hat{I}”t\sim 100$ Myr as structures are flattened by pressure smoothing and photoevaporation. By $\hat{I}”t=300$ 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 $d_{\mathrm{eff}}=1-20$ 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.

19 pages, 11 figures, 1 table submitted to ApJ