Electron-Ion Equilibration in the Merging Galaxy Cluster A665

Abstract

Galaxy cluster mergers drive powerful shock fronts that heat the intracluster medium (ICM) and accelerate particles, redistributing the energy in a merger. A665 is one of only a few clusters with such a powerful shock ($\mathcal{M}\sim$3), and it provides a unique opportunity to study the thermalization timescale of the ICM, particularly the electron-ion equilibration timescale. Understanding this timescale is crucial for determining how the energy from the merger is distributed between thermal and nonthermal particle populations. Using $\sim$200 ks of NuSTAR observations, we measure the temperature distribution across the shock to distinguish between two heating models: (1) an instant collisionless model, where ions and electrons are immediately heated at the shock front; and (2) a collisional model, where electrons are initially adiabatically compressed at the shock and subsequently equilibrate with the ions over $\sim$100 Myr. Our measurements favor the delayed-equilibration model, suggesting that electrons do not immediately reach thermal equilibrium with the ions at the shock front and instead equilibrate over $t_{eq}=(4.0\pm 3.4)\times 10^8$ yr. Additionally, our temperature measurements indicate that the Mach number may be lower than previously estimated ($\mathcal{M}=2.8\pm 0.7$), suggesting that the shock strength has been overestimated in past studies. These results add to our understanding of the microphysics governing how thermal energy is distributed in diffuse plasmas like the ICM, with implications for galaxy cluster evolution, large-scale structure formation, and cosmology.

Published in ApJ - Oct 6, 2025

References in corpus (22)

• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints
• Diffuse Radio Emission from Galaxy Clusters
• NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission
• Chandra Temperature Maps for Galaxy Clusters with Radio Halos
• Physics of the Merging Clusters Cygnus A, A3667, and A2065
• Merger shocks in galaxy clusters A665 and A2163 and their relation to radio halos
• The NewAthena mission concept in the context of the next decade of X-ray astronomy
• The Planck clusters in the LOFAR sky. I. LoTSS-DR2: new detections and sample overview
• Shock fronts, electron-ion equilibration and ICM transport processes in the merging cluster Abell 2146
• Overview of the Advanced X-ray Imaging Satellite (AXIS)
• On the electron-ion temperature ratio established by collisionless shocks
• Calibrating the Planck Cluster Mass Scale with CLASH
• Galaxy clusters enveloped by vast volumes of relativistic electrons
• Effects of the Non-Equipartition of Electrons and Ions in the Outskirts of Relaxed Galaxy Clusters
• A Strong Merger Shock in Abell 665
• A Merger Scenario for the Dynamics of Abell 665
• Non-Equilibrium Electrons in the Outskirts of Galaxy Clusters
• Shocking Features in the Merging Galaxy Cluster RXJ0334.2-0111
• An ALMA+ACA measurement of the shock in the Bullet Cluster
• Bow shock in merging cluster A520: the edge of the radio halo and the electron-ion equilibration timescale
• A Systematic Comparison of Galaxy Cluster Temperatures Measured with NuSTAR and Chandra