A unique, distant cluster pre-merger

Galaxy cluster mergers are excellent laboratories for studying numerous physical processes. Different phases of the merging process reveal different aspects of these fascinating systems. For instance, the pre-merger state can help us to understand the conditions under which the cluster gas interaction starts, the effects of the latter on X-ray cluster properties, and the biases that such systems inflict on cosmological analyses. We use XMM-Newton observations to study the SPT-CLJ2228−5828 cluster system, which constitutes a pre-merger system. It is a high-redshift (z=0.78) cluster pair that was believed to be a dissociative cluster post-merger when studied only by SPT via its SZ signal. It exhibits two major, widely-separated centers in its mass distribution, identified by HST weak lensing observations. With the use of X-rays however, it is revealed that SPT-CLJ2228−5828 consists of two closeby clusters of similar mass in a pre-merging state. This strongly demonstrates the need for high-resolution X-ray observations when we study such high-redshift complex systems. Performing a spectroscopic analysis, we fully characterize the two cluster members of the merger along with the gas bridge that connects them. We check the consistency of the numerous measured X-ray cluster properties with several scaling relations at the high-z regime. This helps us understand the kind of biases that such systems might introduce to scaling relation studies when unidentified. SPT-CLJ2228−5828 is (to our knowledge) the first such high-z pre-merger consisting of equally-sized clusters to be studied. Due to its observational power, Athena will reveal many more such systems at even larger distances. This will help us better understand the cluster formation processes in the early Universe, the merging dynamics between clusters of different masses, and the evolution of these merging systems with redshift