Cosmic Reionization On Computers: Numerical and Physical Convergence

TL;DR

This paper investigates the convergence of reionization simulations, demonstrating that while full convergence is computationally demanding, a weak convergence correction can approximate the results with reasonable accuracy for many galaxy properties.

Contribution

The paper introduces a weak convergence correction factor in the star formation recipe to improve the accuracy of finite resolution reionization simulations.

Findings

Simulations converge in space and mass but slowly.

A 20% precision is achievable for star formation histories.

Weak convergence correction approximates fully converged results.

Abstract

In this paper I show that simulations of reionization performed under the Cosmic Reionization On Computers (CROC) project do converge in space and mass, albeit rather slowly. A fully converged solution (for a given star formation and feedback model) can be determined at a level of precision of about 20%, but such a solution is useless in practice, since achieving it in production-grade simulations would require a large set of runs at various mass and spatial resolutions, and computational resources for such an undertaking are not yet readily available. In order to make progress in the interim, I introduce a weak convergence correction factor in the star formation recipe, which allows one to approximate the fully converged solution with finite resolution simulations. The accuracy of weakly converged simulations approaches a comparable, ~20% level of precision for star formation histories of individual galactic halos and other galactic properties that are directly related to star formation rates, like stellar masses and metallicities. Yet other properties of model galaxies, for example, their HI masses, are recovered in the weakly converged runs only within a factor of two.