Radio emission from a massive node of the cosmic web

Context. The recent detection of radio emission extending beyond the scales typically associated with radio halos challenges our understanding of how energy is transferred to the non-thermal components on the outskirts of galaxy clusters, suggesting the crucial role of mass accretion processes. So far, discoveries have relied on the visual identification of prominent clusters within limited samples. Today, machine learning promises to automatically identify an increasing number of such sources in wide-area radio surveys. Aims. We aim to understand the nature of the diffuse radio emission surrounding the massive galaxy cluster PSZ2 G083.29-31.03, at z = 0.412, already known to host a radio halo. Our investigation was triggered by Radio U-Net, a novel machine learning algorithm for detecting diffuse radio emission that was previously applied to the Low Frequency Array (LOFAR) Two Meter Sky Survey (LoTSS). Methods. We re-processed LoTSS (120–168 MHz) data and analysed archival XMM-Newton (0.7–1.2 keV) observations. We also analysed optical and near-infrared data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys and assessed the mass distribution with weak-lensing analysis based on archival Subaru observations. Results. We report the discovery of large-scale diffuse radio emission around PSZ2 G083.29-31.03, with a projected largest linear size of 5 Mpc at 144 MHz. The radio emission is aligned with the thermal X-ray emission and the distribution of galaxies, unveiling the presence of two low-mass systems, at similar redshifts on either side of the central cluster. The weak lensing analysis supports this scenario, demonstrating the presence of an extended and complex mass distribution. Conclusions. We propose to interpret the two faint radio sources as connected to the central cluster, illuminating the presence of two substructures merging into a massive node of the cosmic web. However, because of uncertainties in redshift and mass estimates, combined with the low resolution required to detect these sources, classification of the two sources as independent radio halos associated with nearby low-mass clusters or even as a mixture of different types of diffuse radio emission cannot be definitively ruled out.