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.
Highlights
Cluster Outskirts as Revealed by ShocksDespite the oft-quoted phrase of galaxy clusters being the largest virialized objects in the universe, at no stage of their evolution are galaxy clusters truly isolated systems
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
Shock measurements for radio relics in the cluster outskirts: for both the nearest radio relic cluster (Coma) using Planck data [15], and for the most distant radio relic cluster (ACT-CL J0102−4915, or “El Gordo”) using ALMA data [16]. In this conference proceeding article, we briefly review these results to illustrate how the joint modeling of the thermal (SZ effect and X-ray) and the non-thermal signals can provide a consistent picture of the shock Mach number and the physical conditions of the intracluster medium (ICM) in the cluster outskirts
Summary
Despite the oft-quoted phrase of galaxy clusters being the largest virialized objects in the universe, at no stage of their evolution are galaxy clusters truly isolated systems. In addition to a continuous matter accretion taking place at the outer boundaries, clusters grow in mass through dramatic merger events which generate low Mach-number shocks in the hot intracluster medium (ICM). We have made the first SZ shock measurements for radio relics in the cluster outskirts: for both the nearest radio relic cluster (Coma) using Planck data [15], and for the most distant radio relic cluster (ACT-CL J0102−4915, or “El Gordo”) using ALMA data [16] In this conference proceeding article, we briefly review these results to illustrate how the joint modeling of the thermal (SZ effect and X-ray) and the non-thermal (radio synchrotron) signals can provide a consistent picture of the shock Mach number and the physical conditions of the ICM in the cluster outskirts. We start by describing how the SZ effect can potentially influence the relic flux measurements at GHz frequencies (1–30 GHz), giving the impression of a rapidly steepening synchrotron spectrum [17]
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