Chandra observation of the merging cluster A168:: A late stage in the evolution of a cold front

We present Chandra observations of the cool cluster A168, for which previous X-ray imaging and optical studies indicated a merger of two subclusters nearly in the plane of the sky. We derive a temperature map for A168, which shows that the merger has proceeded beyond the core passage and is near subcluster turnaround. It also reveals an unusual feature: the gas core of one of the subclusters forms a tonguelike structure extending ahead (in the direction of motion) of the subcluster center. The coolest cluster gas is found in a crescent-shaped region at the tip of this tongue and forms a cold front in pressure equilibrium with the external gas. In contrast with this feature's forward location, previously observed merger cold fronts (e.g., A3667, 1E 0657-56) lagged behind their host subclusters, as expected in the presence of ram pressure. We propose that A168 illustrates a much later stage in the evolution of a cold front, when its host subcluster approaches the apocenter of the merger orbit where the ram pressure on its gas drops sharply. As a result, a large chunk of the subcluster gas slingshots past the dark matter center, becomes unbound from the subcluster, and expands adiabatically, as seen in some recent hydrodynamic simulations.