Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps

TL;DR

This study uses numerical simulations to explore how collisionless plasma instabilities influence magnetic field structures and Faraday Rotation signals in the intracluster medium, revealing distinctive statistical signatures.

Contribution

It introduces a collisionless MHD model to analyze turbulence in the ICM and identifies unique imprints of pressure anisotropy on magnetic and Faraday Rotation maps.

Findings

Collisionless effects cause smaller correlation lengths in magnetic fields.

Pressure anisotropy leaves detectable signatures in Faraday Rotation maps.

Differences are most prominent under ICM-like conditions.

Abstract

Observations of the intracluster medium (ICM) in galaxy clusters suggest for the presence of turbulence and the magnetic fields existence has been proved through observations of Faraday Rotation and synchrotron emission. The ICM is also known to be filled by a rarefied weakly collisional plasma. In this work we study the possible signatures left on Faraday Rotation maps by collisionless instabilities. For this purpose we use a numerical approach to investigate the dynamics of the turbulence in collisionless plasmas based on an magnetohydrodynamical (MHD) formalism taking into account different levels of pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and trans/supersonic regimes, one of them representing the fiducial conditions corresponding to the ICM. From the simulated models we compute Faraday Rotation maps and analyze several statistical indicators in order to characterize the magnetic field structure and compare the results obtained with the collisionless model to those obtained using standard collisional MHD framework. We find that important imprints of the pressure anisotropy prevails in the magnetic field and also manifest in the associated Faraday Rotation maps which evidence smaller correlation lengths in the collisionless MHD case. These points are remarkably noticeable for the case mimicking the conditions prevailing in ICM. Nevertheless, in this study we have neglected the decrease of pressure anisotropy due to the feedback of the instabilities that naturally arise in collisionless plasmas at small scales. This decrease may not affect the statistical imprint differences described above, but should be examined elsewhere.