Radio Relics and Halos

Radio relics and radio halos are diffuse, non-thermal radio emission structures found in galaxy clusters: halos fill the cluster core volume while relics appear as elongated arcs at cluster peripheries near merger shock fronts. Both are powered by synchrotron emission from relativistic electrons in the intracluster magnetic field, but their precise acceleration mechanisms remain debated. Radio halos require in-situ reacceleration of electrons throughout the cluster volume — most likely by turbulence driven by cluster mergers — while radio relics are associated with merger shocks that accelerate electrons via diffuse shock acceleration. ΛCDM-based models predict that radio halos and relics should be exclusively associated with actively merging clusters, yet several cases of halos in apparently relaxed clusters and relics with unusually high Mach numbers inconsistent with the thermal state of the surrounding plasma create tension with the standard picture.

Successive Collision Theory explains the properties of radio relics and halos through the angular momentum inheritance mechanism and gravitational superposition. In SCT, galaxy clusters assembled from collision debris that carried organized angular momentum, producing cluster-scale structures with coherent rotation and velocity shear fields that persist to the present. This residual organized motion — distinct from the turbulence generated by discrete mergers in ΛCDM — continuously stirs the intracluster medium and reaccelerates relativistic electrons even in clusters that appear morphologically relaxed. The halos found in relaxed clusters in SCT are maintained by this angular-momentum-driven turbulence rather than requiring a recent major merger, explaining their presence in seemingly undisturbed systems.

Radio relics with anomalously high Mach numbers are explained in SCT through the gravitational superposition contribution to shock dynamics. The overlapping gravitational fields of nested comoving frames add effective pressure support to the intracluster medium beyond what the thermal electron temperature implies, meaning that shocks propagating through the ICM encounter a medium with higher effective sound speed than thermal measurements suggest. When the Mach number is derived from the temperature jump across the shock using only the thermal sound speed, the result overestimates the actual fluid Mach number — or equivalently, the shock appears more powerful than the thermal state of the pre-shock gas would predict. This apparent Mach number excess is a direct observational signature of the gravitational superposition term in the ICM pressure budget, and SCT predicts it to be strongest in clusters embedded in the densest nodes of the cosmic web where frame superposition is deepest.