Chandra Observation of MS 1455.0+2232: cold fronts in a massive cooling flow cluster?

Abstract

We present the Chandra observation of the cluster of galaxies MS 1455.0+2232. From previous ASCA and ROSAT observations, this cluster was identified as a ``relaxed'' cluster that hosts one of the most massive cooling flows detected. With higher angular resolution, the Chandra X-ray image shows the presence of two surface brightness edges on opposite sides of the X-ray peak: the first at 190 kpc to the north and the second at 450 kpc to the south. Even though the low exposure of this observation limits our ability to constrain the temperature jump across both edges, we show that the northern edge is likely to be a ``cold front'' similar to others observed recently by Chandra in the clusters A2142, A3667, RX J1720.1+2638, and A2256. The observed cold front is most likely produced by the motion, from south to north, of a group-size dark matter halo. The most natural explanation for the presence of this observed moving subclump is that MS 1455.0+2232 is a merger cluster in the very last stage before it becomes fully relaxed. This scenario, however, appears to be unlikely as the cluster shows no further sign of ongoing merger. Moreover, it is not clear if a massive cooling flow could have survived this kind of merger. We propose an alternative scenario in which, as for RX J1720.1+2638, MS 1455.0+2232 is the result of the hierarchical collapse of two co-located density perturbations, the first a group-scale perturbation collapse followed by a second cluster-scale perturbation collapse that surrounded, but did not destroy, the first. We suggest that a cooling flow may have begun inside the already collapsed group-scale perturbation and may have been later amplified by the gas compression induced by the infall of the overlying main cluster mass.