Role of magnetic field strength and numerical resolution in simulations of the heat-flux driven buoyancy instability

TL;DR

This study investigates how magnetic field strength and numerical resolution influence the heat-flux driven buoyancy instability in galaxy cluster cores, revealing conditions under which conductive heat flux can be sustained despite HBI effects.

Contribution

The paper demonstrates that intermediate magnetic field strengths can lead to stable filament formation, allowing sustained heat flux, which contrasts with previous results showing flux suppression.

Findings

Weak initial fields lead to horizontal wrapping of magnetic fields.

Intermediate fields form stable filaments that maintain 10-25% of Spitzer heat flux.

Simulation resolution impacts the observed behavior of HBI and magnetic field configurations.

Abstract

The role played by magnetic fields in the intracluster medium (ICM) of galaxy clusters is complex. The weakly collisional nature of the ICM leads to thermal conduction that is channelled along field lines. This anisotropic heat conduction profoundly changes the stability of the ICM atmosphere, with convective stabilities being driven by temperature gradients of either sign. Here, we employ the Athena magnetohydrodynamic code to investigate the local non-linear behavior of the heat-flux driven buoyancy instability (HBI), relevant in the cores of cooling-core clusters where the temperature increases with radius. We study a grid of 2-d simulations that span a large range of initial magnetic field strengths and numerical resolutions. For very weak initial fields, we recover the previously known result that the HBI wraps the field in the horizontal direction thereby shutting off the heat flux. However, we find that simulations which begin with intermediate initial field strengths have a qualitatively different behavior, forming HBI-stable filaments that resist field-line wrapping and enable sustained vertical conductive heat flux at a level of 10--25% of the Spitzer value. While astrophysical conclusions regarding the role of conduction in cooling cores require detailed global models, our local study proves that systems dominated by HBI do not necessarily quench the conductive heat flux.