An X-ray method for detecting substructure in galaxy clusters - Application to Perseus, A2256, Centaurus, Coma, and Sersic 40/6

Abstract

We use the moments of the X-ray surface brightness distribution toconstrain the dynamical state of a galaxy cluster. Using X-rayobservations from the Einstein Observatory IPC, we measure the firstmoment FM, the ellipsoidal orientation angle, and the axial ratio at asequence of radii in the cluster. We argue that a significant variationin the image centroid FM as a function of radius is evidence for anonequilibrium feature in the intracluster medium (ICM) densitydistribution. In simple terms, centroid shifts indicate that the centerof mass of the ICM varies with radius. This variation is a tracer ofcontinuing dynamical evolution. For each cluster, we evaluate thesignificance of variations in the centroid of the IPC image by computingthe same statistics on an ensemble of simulated cluster images. Inproducing these simulated images we include X-ray point source emission,telescope vignetting, Poisson noise, and characteristics of the IPC.Application of this new method to five Abell clusters reveals that thecore of each one has significant substructure. In addition, we findsignificant variations in the orientation angle and the axial ratio forseveral of the clusters.