Discovery of a dissociative galaxy cluster merger with large physical separation

We present DLSCL J0916.2+2951 (z = 0.53), a newly discovered major cluster merger in which the collisional cluster gas has become dissociated from the collisionless galaxies and dark matter (DM). We identified the cluster using optical and weak-lensing observations as part of the Deep Lens Survey. Our follow-up observations with Keck, Subaru, Hubble Space Telescope, and Chandra show that the cluster is a dissociative merger and constrain the DM self-interaction cross-section σ_DM m ^-1 _DM ≲ 7cm² g^-1. The system is observed at least 0.7 ± 0.2Gyr since first pass-through, thus providing a picture of cluster mergers 2-5times further progressed than similar systems observed to date. This improved temporal leverage has implications for our understanding of merging clusters and their impact on galaxy evolution.