Cosmic Reionization On Computers: Statistics, Physical Properties and Environment of Lyman Limit Systems at $z\sim6$

Abstract

Lyman limit systems (LLSs) are dense hydrogen clouds with high enough HI column densities to absorb Lyman continuum photons emitted from distant quasars. Their high column densities imply an origin in dense environments; however, the statistics and distribution of LLSs at high redshifts still remain uncertain. In this paper, we use self-consistent radiative transfer cosmological simulations from the "Cosmic Reionization On Computers" (CROC) project to study the physical properties of LLSs at the tail end of cosmic reionization at $z\sim6$. We generate 3000 synthetic quasar sightlines to obtain a large number of LLS samples in the simulations. In addition, with the high physical fidelity and resolution of CROC, we are able to quantify the association between these LLS samples and nearby galaxies. Our results show that the fraction LLSs spatially associated with nearby galaxies is increasing with the HI column density. Moreover, we find that LLSs that are not near any galaxy typically reside in filamentary structures connecting neighboring galaxies in the intergalactic medium (IGM). This quantification of the distribution and associations of LLSs to large scale structures informs our understanding of the IGM-galaxy connection during the Epoch of Reionization, and provides a theoretical basis for interpreting future observations.