Features of collisionless turbulence in the intracluster medium from simulated Faraday rotation maps II: the effects of instabilities feedback

TL;DR

This study investigates how micro-scale plasma instabilities and anisotropy relaxation affect Faraday RM map statistics in the ICM, showing that rapid relaxation aligns collisionless models with collisional MHD results, supporting their use in modeling.

Contribution

It extends previous collisionless MHD turbulence analysis by including anisotropy development and micro-instability effects, clarifying their impact on RM map statistics.

Findings

Fast anisotropy relaxation yields RM statistics similar to collisional MHD.

Slow relaxation causes reduced dispersion and steeper power spectra.

Collisionless MHD models are valid if anisotropy is rapidly limited by instabilities.

Abstract

Statistical analysis of Faraday Rotation Measure (RM) maps of the intracluster medium (ICM) of galaxy clusters provides a unique tool to evaluate some spatial features of the magnetic fields there. Its combination with numerical simulations of magnetohydrodynamic (MHD) turbulence allows the diagnosis of the ICM turbulence. Being the ICM plasma weakly collisional, the thermal velocity distribution of the particles naturally develops anisotropies as a consequence of the large scale motions and the conservation of the magnetic moment of the charged particles. A previous study (Paper I) analyzed the impact of large scale thermal anisotropy on the statistics of RM maps synthesized from simulations of turbulence; these simulations employed a collisionless MHD model which considered a tensor pressure with uniform anisotropy. In the present work, we extend that analysis to a collisionless MHD model in which the thermal anisotropy develops according to the conservation of the magnetic moment of the thermal particles. We also consider the effect of anisotropy relaxation caused by the micro-scale mirror and firehose instabilities. We show that if the relaxation rate is fast enough to keep the anisotropy limited by the threshold values of the instabilities, the dispersion and power spectrum of the RM maps are indistinguishable from those obtained from collisional MHD. Otherwise, there is a reduction in the dispersion and steepening of the power spectrum of the RM maps (compared to the collisional case). Considering the first scenario, the use of collisional MHD simulations for modeling the RM statistics in the ICM becomes better justified.