Simulation Study of Binary Mergers of Galaxy Clusters I: Properties of Merger Shocks and Radio Emission

TL;DR

This study uses advanced 3D MHD simulations to explore how galaxy cluster mergers generate shocks and radio relics, revealing complex structures and spectral features influenced by turbulence, magnetic fields, and cosmic-ray physics.

Contribution

It introduces detailed numerical methods for simulating merger shocks, cosmic-ray acceleration, and radio emission, providing new insights into the physics of radio relics in galaxy clusters.

Findings

Axial shocks exhibit higher Mach numbers ahead of the heavier subcluster.

Turbulent magnetic fields create patchy radio surface brightness structures.

Radio spectra deviate from simple power-law expectations due to turbulence and shock curvature.

Abstract

We investigate binary mergers of galaxy clusters and the resulting radio relics using three-dimensional simulations. The initial setup consists of two idealized, spherical subclusters with a mass ratio below three, each permeated by turbulent magnetic fields, and we follow their mergers with a high-order accurate magnetohydrodynamic (MHD) code. In parallel, we track the acceleration of cosmic-ray electrons (CRe) via diffusive shock acceleration (DSA) at merger-driven shocks, together with radiative cooling and Fermi-II (turbulent) acceleration in the postshock region, employing a high-order Fokker-Planck solver. Synchrotron emission is computed from the simulated CRe distribution and magnetic fields. In this paper, we detail these numerical approaches and present the first results obtained with them. Two prominent axial shocks emerge along the merger axis; the shock ahead of the heavier subcluster systematically attains a higher Mach number, although it is more compact, than that ahead of the lighter subcluster. Turbulent magnetic fields--both inherited from the initial condition and amplified during the merger--produce patchy, fine-scale structures in the radio surface brightness. Because of the combined effects of turbulent acceleration, spatially nonuniform magnetic fields, and the curved geometry of merger shocks, the volume-integrated radio spectra show deviations from the canonical power-law steepening expected for a planar shock with a uniform field. Reacceleration of preexisting fossil CRe enhances the surface brightness. Our results highlight the coupled roles of merger dynamics, MHD turbulence, and CRe physics in shaping up the observed properties of radio relics in cluster outskirts.