SPH simulations of magnetic fields in galaxy clusters

TL;DR

This paper uses cosmological MHD simulations to study how initial magnetic fields evolve in galaxy clusters, demonstrating that shear flows and cluster collapse significantly amplify and shape magnetic fields consistent with observations.

Contribution

It introduces a combined Grape and SPH simulation approach to model magnetic field evolution in galaxy clusters, revealing the importance of shear flows and initial field strength.

Findings

Magnetic fields are amplified by about three orders of magnitude in cluster cores.

Different initial magnetic configurations produce similar final fields.

Initial fields of ~10^-9 G at redshift 15 reproduce observed Faraday rotation measures.

Abstract

We perform cosmological, hydrodynamic simulations of magnetic fields in galaxy clusters. The computational code combines the special-purpose hardware Grape for calculating gravitational interaction, and smooth-particle hydrodynamics for the gas component. We employ the usual MHD equations for the evolution of the magnetic field in an ideally conducting plasma. As a first application, we focus on the question what kind of initial magnetic fields yield final field configurations within clusters which are compatible with Faraday-rotation measurements. Our main results can be summarised as follows: (i) Initial magnetic field strengths are amplified by approximately three orders of magnitude in cluster cores, one order of magnitude above the expectation from spherical collapse. (ii) Vastly different initial field configurations (homogeneous or chaotic) yield results that cannot significantly be distinguished. (iii) Micro-Gauss fields and Faraday-rotation observations are well reproduced in our simulations starting from initial magnetic fields of \~ 10^-9 G strength at redshift 15. Our results show that (i) shear flows in clusters are crucial for amplifying magnetic fields beyond simple compression, (ii) final field configurations in clusters are dominated by the cluster collapse rather than by the initial configuration, and (iii) initial magnetic fields of order 10^-9 G are required to match Faraday-rotation observations in real clusters.