2025ApJ...987L..40H

Clusters of galaxies host spectacular diffuse radio sources, extending over scales from 100 kpc to several Mpcs. These sources, with extremely faint surface brightness (μJy/arcsec² level), are not tied to individual galaxies but trace synchrotron emission from large-scale magnetic fields and relativistic particles within the intracluster environment. Here, we report the discovery of a candidate radio minihalo in SpARCS104922.6+564032.5, the most distant cool-core galaxy cluster identified to date at z = 1.709, using deep LOFAR 120-168 MHz observations. We show that this emission originates from diffuse cluster-associated processes rather than unresolved active galactic nuclei or star-forming galaxies. The diffuse radio emission coincides spatially with the X-ray emission of the hot intracluster medium and has a radio power of ${P}_{150\,{\rm{MHz}}}=49.{8}_{-11.7}^{+14.7}\times 1{0}^{24}$ W Hz^–1, exhibiting striking similarities to low-redshift radio minihalos. This discovery doubles the redshift of previously known minihalos, challenging models of inverse Compton losses and indicating the presence of strong magnetic fields, enhanced turbulence in high-redshift clusters, or active hadronic processes that require a cosmic-ray-to-thermal-energy ratio of 0.07 within 200 kpc, assuming a clumped distribution with spatial correlations among the gas, cosmic rays, and magnetic field that partially compensate for cosmological redshift dimming. It further implies that magnetic fields are efficiently amplified to ∼10 μG levels within an Mpc³ volume during the epoch of cluster formation before z ∼ 2. These findings provide critical insights into high-redshift cluster physics and emphasize the transformative potential of next-generation radio surveys, such as those with the Square Kilometre Array and next-generation Very Large Array (ngVLA), in exploring the early evolution of galaxy clusters.