The MeerKAT Massive Distant Clusters Survey: Detection of Diffuse Radio Emission in Galaxy Clusters at z > 1

Diffuse, low surface-brightness radio emission in merging galaxy clusters provides insights into cosmic structure formation, the growth of magnetic fields, and turbulence. This paper reports a search for diffuse radio emission in a pilot sample of six high-redshift (1.01 < z < 1.31) galaxy clusters from the MeerKAT Massive Distant Cluster Survey (MMDCS). These six clusters are selected as the most massive (M500c = 6.7\, - 8.5 × 10¹⁴~ M) systems based on their Sunyaev-Zel’dovich mass from the full MMDCS sample of 30 ACT DR5 clusters, and were observed first to explore the high-mass, high-redshift regime. Diffuse radio emission is confidently detected in four clusters and tentatively identified in two, with k-corrected radio powers scaled to 1.4 GHz ranging from (0.46 ± 0.16) to (4.51 ± 1.68) × 1024 WHz−1 and linear sizes between 0.47 and 1.08 Mpc. Combining Chandra X-ray data with MeerKAT radio data, we find that 80% of clusters with X-ray observations exhibit disturbed morphologies indicative of mergers. These z > 1 galaxy clusters scatter around the established radio power-mass scaling relation observed at lower redshifts, supporting turbulent re-acceleration models in high-redshift mergers. However, their radio spectra are predicted to steepen (α < −1.5) due to enhanced inverse Compton losses in the cosmic microwave background, rendering them under-luminous at 1.4 GHz and placing them below the correlation. Our results demonstrate that merger-driven turbulence can sustain radio halos even at z > 1 while highlighting MeerKAT’s unique ability to probe non-thermal processes in the early universe.