2025ApJ...992...88R

We present new, high frequency radio observations of the merging galaxy clusters PLCK G287.0+32.9, A2744, and Bullet. These clusters are known to host ∼Mpc scale sources, known as radio halos, which are formed by the acceleration of cosmic rays by turbulence injected into the intracluster medium during cluster mergers. Our new images reveal previously undetected faint outermost regions of halos, extending to over 2 Mpc. This discovery highlights the presence of radio halos with large extents at high frequencies and suggests that their observable size depends on a combination of the observation sensitivity and uv-coverage, as well as their radio power. We additionally compare the properties of these three clusters with MACS J0717+3745 and A2142, both of which are known to host prominent large radio halos. Remarkably, all five halos, despite their exceptionally large extents, exhibit properties similar to other classical halos: their radial profiles are described by a single-component exponential fit, they show radial spectral index steepening, and have an average radio emissivity of about 10^–42 erg s^–1 cm^–3 Hz^–1. Our results demonstrate that radio halos can extend to the cluster's periphery, without the transition to an observationally distinguishable different halo component in the outermost regions. Our findings also highlight that careful subtraction of unrelated sources embedded in the halo is necessary to measure the radio surface brightness accurately, since incomplete subtraction can introduce an apparent secondary component in the peripheral regions.