2025A&A...698L..17S

In the current theoretical paradigm, high-redshift radio haloes are expected to be scarce due to inverse Compton energy losses and redshift dimming, which cause them to be intrinsically faint. This low occurrence fraction is predicted by cosmic ray electron turbulent re-acceleration models. To date, only a handful of radio haloes have been detected at redshift z > 0.8. We report the MeerKAT detection of a radio halo hosted by the galaxy cluster ACT-CL J0329.2-2330 that has a mass of M$ _{\mathrm{500c}} = 9.7^{+1.7}_{-1.6}\times 10^{14} $ M_⊙ and is at z = 1.23, making it the highest redshift halo detected thus far. Using L-band and UHF-band observations, we derive a radio halo spectral index of $ \alpha^{1.3\,\mathrm{GHz}}_{0.8\,\mathrm{GHz}} $ = 1.3 ± 0.4 and a radio power of P_1.4 GHz_ = (4.4 ± 1.5)×10²⁴ W Hz^–1 that is similar to the typical radio power of haloes hosted by low-redshift galaxy clusters. This result further confirms that there is rapid magnetic field amplification in galaxy clusters at high redshifts.