An efficient hybrid method to produce high resolution large volume dark matter simulations for semi-analytic models of reionisation

TL;DR

This paper introduces a hybrid Monte Carlo method to extend the mass resolution of large volume N-body simulations, enabling more accurate semi-analytic models of cosmic reionisation by resolving faint galaxies.

Contribution

The authors develop a novel Monte Carlo algorithm to enhance mass resolution and evolve halo positions, producing large-volume halo catalogues that improve reionisation modeling accuracy.

Findings

Halo mass function matches high-resolution simulations

Consistent two-point correlation functions with analytic predictions

Improved predictions of stellar mass functions and neutral fractions

Abstract

Resolving faint galaxies in large volumes is critical for accurate cosmic reionisation simulations. While less demanding than hydrodynamical simulations, semi-analytic reionisation models still require very large N-body simulations in order to resolve the atomic cooling limit across the whole reionisation history within box sizes $\gtrsim 100 \, h^{-1} {\rm Mpc}$. To facilitate this, we extend the mass resolution of N-body simulations using a Monte Carlo algorithm. We also propose a method to evolve positions of Monte Carlo halos, which can be an input for semi-analytic reionisation models. To illustrate, we present an extended halo catalogue that reaches a mass resolution of $M_\text{halo} = 3.2 \times 10^7 \, h^{-1} \text{M}_\odot$ in a $105 \, h^{-1} {\rm Mpc}$ box, equivalent to an N-body simulation with $\sim 6800^3$ particles. The resulting halo mass function agrees with smaller volume N-body simulations with higher resolution. Our results also produce consistent two-point correlation functions with analytic halo bias predictions. The extended halo catalogues are applied to the \textsc{meraxes} semi-analytic reionisation model, which improves the predictions on stellar mass functions, star formation rate densities and volume-weighted neutral fractions. Comparison of high resolution large volume simulations with both small volume or low resolution simulations confirms that both low resolution and small volume simulations lead to reionisation ending too rapidly. Lingering discrepancies between the star formation rate functions predicted with and without our extensions can be traced to the uncertain contribution of satellite galaxies.