Detection of a Pre-merger Shock in the Galaxy Cluster Abell 98

We report on the first unambiguous detection of an axial pre-merger shock, in the merging galaxy cluster Abell 98. The shock front resides in an inter-cluster filament, wherein we also find evidence for cool, diffuse gas, the properties of which are consistent with the dense end of the warm-hot intergalactic medium (WHIM). Similar potential WHIM emission is detected outside of the inter-cluster filament, along the merger axis, consistent with the presence of a large scale structure filament aligned with the merger axis. A detailed analysis of the shock front allows constraints to be placed on the mode of electron heating in the post-shock region (instant versus collisional equilibration), for the first time in an early-stage, pre-merger shock. Such detailed studies of the pre-merger phase of the growth of galaxy clusters are relatively rare.