Chandra observation of a cold front in Abell 2554

Abstract

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 similar or equal to 60 h(-1) kpc from the cluster core. The thermodynamical profiles across the edge are ruling out the shock scenario. The temperature jump (from similar to 6 keV to similar to 10 keV), and pressure equilibrium (P-0/P-1 = 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 similar to 100 kpc in front of the cold cloud, corresponding an upper limit to the Mach number M-1 similar to 1.1. If the northern substructure 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.