Direct evidence of a major merger in the Perseus cluster

Although the Perseus cluster has often been regarded as an archetypical relaxed galaxy cluster, several lines of evidence, including ancient, large-scale cold fronts, asymmetric plasma morphology, filamentary galaxy distribution and so on, provide a conflicting view of its dynamical state, suggesting that the cluster might have experienced a major merger. However, the absence of a clear merging companion identified so far hampers our understanding of the evolutionary track of the Perseus cluster consistent with these observational features. Here, through careful weak-lensing analysis, we successfully identified the missing subcluster halo (total mass $${M}_{200}={1.70}_{-0.59}^{+0.73}\times {10}^{14}\,{M}_{\odot }$$ ) at the >5σ level centred on NGC 1264, which is located ~430 kpc west of the Perseus main cluster core. Moreover, a significant (>3σ) mass bridge, which is also traced by the cluster member galaxies, is detected between the Perseus main and subclusters, which serves as direct evidence of gravitational interaction. With idealized numerical simulations, we demonstrate that an ~3:1 off-axis major merger can create the cold front observed ~700 kpc east of the main cluster core and generate the observed mass bridge through multiple core crossings. This discovery resolves the long-standing puzzle of Perseus’s dynamical state. The identification via weak lensing of a subcluster halo, connected to the galaxy cluster Perseus by a mass bridge, provides direct evidence of a past major merger, reshaping our understanding of Perseus’s dynamical history.