Towards convergence of turbulent dynamo amplification in cosmological simulations of galaxies

TL;DR

This study compares two refinement strategies in cosmological galaxy simulations to understand their impact on turbulent dynamo amplification of magnetic fields, revealing that a high-resolution uniform grid accelerates magnetic energy growth and aligns with small-scale turbulence results.

Contribution

Introduces a new high-resolution quasi-uniform refinement method that improves magnetic field amplification modeling in galaxy simulations.

Findings

High-resolution uniform grid enhances magnetic energy amplification.

Magnetic energy growth scales with resolution as ∝ D_res^{-1/2}.

Most amplification occurs in the warm interstellar medium phase.

Abstract

Our understanding of the process through which magnetic fields reached their observed strengths in present-day galaxies remains incomplete. One of the advocated solutions is a turbulent dynamo mechanism that rapidly amplifies weak magnetic field seeds to the order of ${\sim}\mu$G. However, simulating the turbulent dynamo is a very challenging computational task due to the demanding span of spatial scales and the complexity of the required numerical methods. In particular, turbulent velocity and magnetic fields are extremely sensitive to the spatial discretisation of simulated domains. To explore how refinement schemes affect galactic turbulence and amplification of magnetic fields in cosmological simulations, we compare two refinement strategies. A traditional quasi-Lagrangian adaptive mesh refinement approach focusing spatial resolution on dense regions, and a new refinement method that resolves the entire galaxy with a high resolution quasi-uniform grid. Our new refinement strategy yields much faster magnetic energy amplification than the quasi-Lagrangian method, which is also significantly greater than the adiabatic compressional estimate indicating that the extra amplification is produced through stretching of magnetic field lines. Furthermore, with our new refinement the magnetic energy growth factor scales with resolution following $\propto \Dres^{-1/2}$, in much better agreement with small-scale turbulent box simulations. Finally, we find evidence suggesting most magnetic amplification in our simulated galaxies occurs in the warm phase of their interstellar medium, which has a better developed turbulent field with our new refinement strategy.