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

TL;DR

This paper investigates the physical properties and evolution of ionizing photon sinks in the intergalactic medium after reionization, revealing their significant role in early universe opacity and how they change over hundreds of millions of years.

Contribution

It provides detailed simulations of self-shielding systems, characterizing their masses, sizes, and evolution, highlighting their impact on IGM opacity during reionization.

Findings

Self-shielding systems contribute over half of the opacity shortly after I-front passage.

Structures evolve significantly over 100 Myr, reducing their contribution to less than 10%.

Characteristic masses range from 10^4 to 10^8 solar masses, with sizes of 1-20 ckpc/h.

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 $\Delta t = 10$ 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.