Speaker
Description
Measurements of the ionizing background and the mean free path (MFP) at the end of reionization offer valuable insights into the first generation of sources that ionized the universe. With increasing observational data from this epoch, a better theoretical understanding is imperative to interpret the data accurately.
The CROC simulations are uniquely poised to interpret observational data, with both high spatial resolution to resolve galaxy formation and fully-coupled radiative transfer to authentically simulate the reionization of the intergalactic medium (IGM). We analyze two CROC boxes with distinct reionization histories to gain insights into the uniformity of the ionizing background and MFP of photons. We find that in the late reionization box, where the volume-weighted neutral fraction x_HI drops to 0.5 at z=7.4 and below 0.001 at z=6.4, the ionizing background still displays significant fluctuations (~40%) at z=5. These fluctuations are closely related to the non-uniform distribution of ionizing sources and Lyman Limit Systems (LLSs). Additionally, we investigate the mean free path (MFP) and observe differences across different environments. Notably, the MFP around very massive halos (>1e12 Msun) is significantly smaller than in average regions of the universe, attributed to the abundance of LLSs.
Observations measure the mean free path (MFP) using quasar spectra, where the ionization of Lyman Limit Systems (LLSs) around quasar hosts must be considered. For consistent comparison to observations, we create thousands of mock quasar spectra by post-processing the sightlines centered on massive halos, and assess the accuracy of the observational procedure outlined in Becker+21. Our analysis quantifies the bias in the inferred MFP, highlighting important implications for interpreting observational data and understanding the end stage of reionization.
