A Yin-yang Galaxy Cluster Merger in A1914 Revealed by XRISM

Abstract Hierarchical mergers of galaxy clusters play a key role in converting gravitational energy into thermal and kinetic energy in the local Universe. Understanding this process requires the reconstruction of cluster merger geometry, with careful consideration of projection effects. With its unprecedented spectral resolution, XRISM enables the disentanglement of merging cluster components along the line of sight via X-rays for the first time. In this Letter, we focus on the massive cluster A1914, a puzzling case wherein the galaxy and dark matter (DM) distribution appear to be in tension with the X-ray morphology. We present XRISM observations of A1914 focusing on the velocity structure of the intracluster medium. The Resolve full-array spectrum requires two merging components along the line of sight, with bulk velocities offset by ∼1000 km s −1 and velocity dispersions of ∼200 km s −1 . The subarray maps of flux ratios, bulk velocity, and velocity dispersion show the two components are offset and overlapping in the plane of the sky, consistent with a major (mass ratio ∼3), near line-of-sight merger with a pericenter distance of ∼200 kpc. We conclude that the two subclusters create an overlapping spiral pattern, referred to as a “yin-yang” merger. This scenario is further supported by tailored hydrodynamical simulations of the A1914 merger, demonstrating that this type of merger can broadly reproduce the observed X-ray morphology, gas temperature map, gas velocity maps, DM distribution, and galaxy velocities. This work demonstrates the power of high-resolution X-ray spectroscopy, provided by XRISM, to resolve complex cluster merger geometries.