2019MNRAS.488.2917E

We present the evidence for the existence of sub-structure in the cold front cluster A2554 based on a 20.14 ks Chandra observation. Using centroid shift and X-ray brightness concentration parameters, we confirm that A2554 is a dynamically disturbed system. We detect two dominant structures: a main cluster at z = 0.1108 and a foreground northern sub-structure at z = 0.1082. The analysis reveals an X-ray surface brightness edge at r ≃ 60 h^–1 kpc from the cluster core. The thermodynamical profiles across the edge are ruling out the shock scenario. The temperature jump (from ∼6 keV to ∼10 keV), and pressure equilibrium (P₀/P₁ = 1.01 ± 0.23) across the edge, are consistent with the definition of a cold front with a Mach number M_1=0.94+0.13^_–0.17. We also observed a weak bow-shock at ∼100 kpc in front of the cold cloud, corresponding an upper limit to the Mach number M_1 ∼1.1. If the northern sub-structure was not related to the cold front, we conclude that the transonic motion of the cloud is caused by a merger, which was weak or occurred long ago.