The effect of pressure-anisotropy-driven kinetic instabilities on magnetic field amplification in galaxy clusters

TL;DR

This study investigates how pressure-anisotropy-driven kinetic instabilities influence magnetic field amplification in galaxy clusters, using models of turbulence and merger histories to determine the redshift at which dynamo processes become effective.

Contribution

It introduces a model incorporating kinetic instabilities into the turbulent dynamo framework, revealing the redshift dependence of magnetic field amplification in galaxy clusters.

Findings

Magnetic fields need to start amplifying by redshift z=1.5 to reach equipartition by z=0.

Higher Re_eff models accelerate magnetic field growth and reduce the time to reach equipartition.

Merger trees effectively model magnetic field evolution in weakly collisional plasmas.

Abstract

The intracluster medium (ICM) is the low-density diffuse magnetized plasma in galaxy clusters, which reaches virial temperatures of up to 10^8 K. Under these conditions, the plasma is weakly collisional and therefore has an anisotropic pressure tensor with respect to the local direction of the magnetic field. This triggers very fast, Larmor-scale, pressure-anisotropy-driven kinetic instabilities that alter magnetic field amplification. We study magnetic field amplification through a turbulent small-scale dynamo, including the effects of the kinetic instabilities, during the evolution of a typical massive galaxy cluster. A specific aim of this work is to establish a redshift limit from which a dynamo has to start to amplify the magnetic field up to equipartition with the turbulent velocity field at redshift z=0. We implemented 1D radial profiles for various plasma quantities for merger trees generated with the Modified GALFORM algorithm. We assume that turbulence is driven by successive mergers of dark matter halos and construct effective models for the Reynolds number Re_eff dependence on the magnetic field in three different magnetization regimes, including the effects of kinetic instabilities. The magnetic field growth rate is calculated for the different Re_eff models. The model results in a higher magnetic field growth rate at higher redshift. For all scenarios considered, to reach equipartition at z=0, the amplification of the magnetic field has to start at redshift z_start=1.5 and above. The time to reach equipartition can be significantly shorter, in cases with systematically smaller turbulent forcing scales, and for the highest Re_eff models. Merger trees are useful tools for studying the evolution of magnetic fields in weakly collisional plasmas. They could also be used to constrain the different stages of the dynamo that could be observed by future radio telescopes.