2025MNRAS.542.1544P

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 Clusters Survey (MMDCS). These six clusters are selected as the most massive ($M_{\rm 500c} = 6.7 \!-\! 8.5 \times 10^{14} \, {{\rm M}_{\odot }}$) systems based on their Sunyaev-Zel'dovich mass from the full MMDCS sample of 30 Atacama Cosmology Telescope Data Release 5 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 $(0.46 \pm 0.16)$ to $(4.51 \pm 1.68) \times 10^{24}\, \mathrm{WHz^{-1}}$ and linear sizes between 0.47 and 1.08 Mpc. Combining Chandra X-ray data with MeerKAT radio data, we find that 80 per cent 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 reacceleration models in high-redshift mergers. However, their radio spectra are predicted to steepen ($\alpha < -1.5$) due to enhanced inverse Compton losses in the cosmic microwave background, rendering them underluminous at 1.4 GHz and placing them below the correlation. Our results demonstrate that merger-driven turbulence can sustain radio haloes even at $z > 1$ while highlighting MeerKAT's unique ability to probe non-thermal processes in the early Universe.