Cosmic Reionization on Computers: The Evolution of the Ionizing Background and Mean Free Path

Abstract

Observations of the end stages of reionization indicate that at z ≈ 5–6, the ionizing background is not uniform and the mean free path (MFP) changes drastically. As the MFP is closely related to the distribution of Lyman-limit systems (LLSs) and damped Ly α absorbers, it is important to understand them. In this study, we utilize the Cosmic Reionization on Computers (CROC) simulations, which have both sufficient spatial resolution to resolve galaxy formation and LLSs alongside a fully coupled radiative transfer, to simulate the reionization processes. We analyze two CROC boxes with distinct reionization histories and find that the distributions of the ionizing background in both simulations display significant skewness. Further, the ionizing background in the late-reionization box still displays significant fluctuations (∼40%) at z ≈ 5. We also measure the MFP along sightlines that center on potential quasar hosting halos. The evolution of the MFP measured from these sightlines exhibits a break that coincides with the disappearance of all the neutral islands in the reionization history of each box. In the absence of LLSs, the MFP will be biased high by ≈20% at z ≈ 5. We also compare the MFPs measured in random sightlines. We find that at z ≈ 5, the MFPs measured in sightlines that start from massive halos are systematically smaller by ≈10% compared with the MFPs measured in random sightlines. We attribute this difference to the concentration of dense structures within 1 pMpc of massive halos. Our findings highlight the importance of high-fidelity models in the interpretation of observational measurements.

Keywords

• Reionization
• Intergalactic medium