Cosmic Reionization On Computers III. The Clumping Factor

TL;DR

This paper uses advanced numerical simulations to evaluate how effectively the clumping factor describes recombination rates in the inhomogeneous ionized intergalactic medium during cosmic reionization.

Contribution

It provides a detailed analysis of the clumping factor's accuracy and introduces the concept of a local clumping factor with its scale-dependent variability.

Findings

The IGM's density distribution is highly inhomogeneous but still well-defined.

The clumping factor can be computed with about 20% accuracy due to unvirialized regions.

The local clumping factor varies significantly with spatial scale.

Abstract

We use fully self-consistent numerical simulations of cosmic reionization, completed under the Cosmic Reionization On Computers (CROC) project, to explore how well the recombinations in the ionized IGM can be quantified by the effective "clumping factor." The density distribution in the simulations (and, presumably, in a real universe) is highly inhomogeneous and more-or-less smoothly varying in space. However, even in highly complex and dynamic environments the concept of the IGM remains reasonably well-defined; the largest ambiguity comes from the unvirialized regions around galaxies that are over-ionized by the local enhancement in the radiation field ("proximity zones"). That ambiguity precludes computing the IGM clumping factor to better than about 20\%. We also discuss a "local clumping factor," defined over a particular spatial scale, and quantify its scatter on a given scale and its variation as a function of scale.