Unveiling large scale structure formation in the nearby Universe. The spectacular case of the Shapley Concentration

The Shapley Supercluster (mean redshift z=0.048) has one of the most extensive collections of galaxy clusters in the local Universe. At its heart lies the Shapley Supercluster Core (SSC), consisting of many clusters and groups at various evolutionary stages of merger activity. The SSC is thus a unique test bed to study mass assembly history and the complex kinematics of large-scale structure formation in the Universe. The entire complex spans ~4 deg on the sky, containing an extremely faint ~1 Mpc inter-cluster radio bridge (diffuse synchrotron emission that connects pairs of gravitationally interacting galaxy clusters) and many other intriguing non-thermal diffuse sources that probe the underlying dynamics of this chaotic environment. We aim to compare MeerKAT and uGMRT radio continuum observations and perform the first dedicated spectral study of an intercluster radio bridge to unveil the past and current dynamics of this region and the merger activity of the broader complex. A preliminary integrated spectrum of the bridge between MeerKAT L-band and uGMRT band-3 gives a flatter spectrum than expected, at $\alpha^{\rm 1.28GHz}_{0.4GHz} = 1.1 +/- 0.3$ (S_nu \propto \nu^{-\alpha}), suggesting very mild ongoing in-situ particle (re)-acceleration. However, detailing and tracing the merger history with robust spectral index and ageing maps requires ultra-deep continuum images of uniform sensitivity across the entire complex. Hence, we demonstrate, using DDFacet and other radio interferometric data reduction software, a novel technique of visibility-plane primary-beam-corrected mosaicing up to and including direction-dependent calibration. This technique can be extended to large survey projects such as those planned for the MeerKAT+ and the SKA to, in principle, image the entire radio sky at an unprecedented uniform sensitivity.