The effect of dark matter resolution on the collapse of baryons in high redshift numerical simulations

TL;DR

This study investigates how dark matter particle resolution affects baryonic core formation in high-redshift simulations, highlighting the importance of particle splitting and resolution criteria for accurate results.

Contribution

It introduces particle splitting methods to improve dark matter resolution without noise and establishes a convergence criterion based on baryon-to-dark matter mass ratio.

Findings

Particle splitting enhances resolution without numerical noise.

Convergence achieved when baryonic mass dominates dark matter mass by a factor of over 100.

Lower background fields require lower dark matter particle mass for accurate simulation.

Abstract

We examine the impact of dark matter particle resolution on the formation of a baryonic core in high resolution adaptive mesh refinement simulations. We test the effect that both particle smoothing and particle splitting have on the hydrodynamic properties of a collapsing halo at high redshift (z > 20). Furthermore, we vary the background field intensity, with energy below the Lyman limit (< 13.6 eV), as may be relevant for the case of metal-free star formation and super-massive black hole seed formation. We find that using particle splitting methods greatly increases our particle resolution without introducing any numerical noise and allows us to achieve converged results over a wide range of external background fields. Additionally, we find that for lower values of the background field a lower dark matter particle mass is required. We define the radius of the core as the point at which the enclosed baryonic mass dominates over the enclosed dark matter mass. For our simulations this results in $\rm{R_{core} \sim 5\ pc}$. We find that in order to produce converged results which are not affected by dark matter particles requires that the relationship ${M_{\rm{core}} / M_{\rm{DM}}} > 100.0$ be satisfied, where ${M_{\rm{core}}}$ is the enclosed baryon mass within the core and $M_{\rm{DM}}$ is the minimum dark matter particle mass. This ratio should provide a very useful starting point for conducting convergence tests before any production run simulations. We find that dark matter particle smoothing is a useful adjunct to already highly resolved simulations.