Galaxy cluster outskirts from the thermal SZ and non-thermal synchrotron link

TL;DR

This paper reviews how combined thermal SZ and non-thermal synchrotron observations of galaxy cluster outskirts reveal shock properties, gas thermodynamics, and magnetic fields, with recent SZ-based Mach number measurements of radio relic shocks.

Contribution

It introduces the first use of the SZ effect to measure Mach numbers of radio relic shocks in galaxy clusters, enhancing understanding of shock physics.

Findings

SZ effect can measure shock Mach numbers in galaxy clusters.

Joint thermal and non-thermal modeling improves knowledge of cluster outskirts.

First SZ-based Mach number measurements for Coma and El Gordo clusters.

Abstract

Galaxy cluster merger shocks are the main agent for the thermalization of the intracluster medium and the energization of cosmic ray particles in it. Shock propagation changes the state of the tenuous intracluster plasma, and the corresponding signal variations are measurable with the current generation of X-ray and Sunyaev-Zel'dovich (SZ) effect instruments. Additionally, non-thermal electrons (re-)energized by the shocks sometimes give rise to extended and luminous synchrotron sources known as radio relics, which are prominent indicators of shocks propagating roughly in the plane of the sky. In this short review, we discuss how the joint modeling of the non-thermal and thermal signal variations across radio relic shock fronts is helping to advance our knowledge of the gas thermodynamical properties and magnetic field strengths in the cluster outskirts. We describe the first use of the SZ effect to measure the Mach numbers of relic shocks, for both the nearest (Coma) and the farthest (El Gordo) clusters with known radio relics.