Unveiling a critical epoch in the formation of massive galaxy clusters

Galaxy cluster mergers are the most energetic events in our Universe since the Big Bang. Mergers induce shocks that dissipate energy to the cluster gas by heating. X-ray observations of cluster mergers detect those shock fronts by measuring temperatures and surface brightness jumps across them. Several previous studies detected such shock fronts in merging clusters at a later stage of merging, i.e., after the core passage. When two clusters approach each other for the very first time, simulations predict that each cluster drives a shock that propagates towards another cluster. Such shocks were never observed before, ignoring a critical epoch in the formation of massive galaxy clusters. We report the first unambiguous detection of a shock along the merger axis in the early-stage merging cluster Abell 98 by using 227 ks Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of ~ 2.3. We also report detection of an intercluster gas filament, with a temperature of 1.07 keV, along the merger axis of Abell 98. Our measurement of the gas properties in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm-hot intergalactic medium (WHIM), where it connects to galaxy clusters.