2025A&A...694A.320H

Context. Cosmic filaments are vast, faint structures that connect galaxy clusters, often challenging to detect directly. However, filaments between pre-merger cluster pairs become more visible due to gas heating and compression while the clusters are approaching, enabling detection in X-ray and radio wavelengths. The clusters Abell 3017 and Abell 3016 are located within such a large-scale filament. A prominent X-ray bridge has been detected connecting the two clusters and a potential galaxy group between them.
Aims. The aim of this work is to investigate the existence of a radio bridge in the filament between Abell 3017 and Abell 3016, to explore other diffuse radio structures within this system, and to investigate the origins of these diffuse radio emission.
Methods. We analysed MeerKAT L-band data to study the morphology and spectra of the diffuse radio structures in Abell 3016-Abell 3017. X-ray imaging and spectral analysis were carried out with archival Chandra and XMM-Newton data. Additionally, correlations between radio (I_R) and X-ray surface brightness (I_X) were generated to explore the connections between thermal and non-thermal components in the diffuse radio emission.
Results. We detected a faint radio bridge with an average surface brightness of ∼0.1 μJy arcsec^–2 at 1280 MHz using MeerKAT. It connects Abell 3017 with a potential galaxy group and extends towards Abell 3016, aligning with the X-ray bridge. A high X-ray temperature of 7.09 ± 0.54 keV detected in the bridge region suggests an interaction between Abell 3017 and the group. In Abell 3017, we identified two distinct components of diffuse radio emission: a radio mini-halo and an outer radio halo with a northern extension (N-extension hereafter). The radio surface brightness profile of Abell 3017 shows a steep inner component consistent with other mini-halos, and a faint outer component likely linked to an infalling subcluster. The I_R - I_X diagram indicates superlinear and sub-linear correlations for the mini-halo and N-extension, respectively.
Conclusions. We proposed three plausible explanations for the origin of the radio bridge: (1) it is an inter-cluster radio bridge connecting the two clusters in a filament, enhanced by interactions with the embedded galaxy group; (2) it results from an interaction between Abell 3017 and the galaxy group after their primary apocentric passage, with the group currently falling back towards Abell 3017; (3) it is a cluster radio relic associated with a merger shock, appearing as a bridge due to its face-on orientation. In Abell 3017, the mini-halo is likely powered by gas sloshing, resulting from an offset merger that left the cluster's cool core intact. Turbulence from an infalling subcluster likely contributes to the formation of the outer radio halo.