Origin of radio mini-halos in cool-core clusters

Determine the physical mechanism responsible for the formation of radio mini-halos in cool-core galaxy clusters, distinguishing between turbulent reacceleration of seed cosmic-ray electrons and hadronic production of secondary electrons via inelastic cosmic-ray proton collisions with intracluster medium protons, given that the origin of radio mini-halos remains unclear despite their identification in approximately forty systems.

Background

Radio mini-halos are faint, diffuse synchrotron sources observed in the cores of cool-core galaxy clusters and have been identified in ~40 systems. The radiative lifetime of electrons from central AGN jets appears too short to account for their large extents, motivating two leading explanations: (i) turbulent reacceleration of pre-existing cosmic-ray electrons, potentially driven by sloshing or AGN activity, and (ii) continuous hadronic injection of secondary electrons through cosmic-ray proton interactions with the thermal intracluster medium. Despite extensive observational and theoretical work, a definitive origin has not been established.

This paper reports a candidate radio mini-halo at z=1.709 in SpARCS1049+56, doubling the redshift of known examples and challenging expectations from increased inverse Compton losses at high redshift. The detection underscores the urgency of resolving the origin question to interpret high-redshift observations and the evolution of cluster magnetic fields and cosmic rays.