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For refcode 1993ApJ...413..492M:
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1993ApJ...413..492M AN X-RAY METHOD FOR DETECTING SUBSTRUCTURE IN GALAXY CLUSTERS: APPLICATION TO PERSEUS, A2256, CENTAURUS, COMA, AND SERSIC 40/6 JOSEPH J. MOHR, DANIEL G. FABRICANT, AND MARGARET J. GELLER Harvard-Smithsonian Center for Astrophysics, MS-10, 60 Garden Street, Cambridge, MA 02138 Received 1992 September 30; accepted 1993 February 24 ABSTRACT We use the moments of the X-ray surface brightness distribution to constrain the dynamical state of a galaxy cluster. Using X-ray observations from the Einstein Observatory Imaging Proportional Counter (IPC), we measure the first moment M_1_(r^bar^), the ellipsoidal orientation angle {theta}_2_(r^bar^), and the axial ratio n(r^bar^) at a sequence of radii in the cluster. We argue that a significant variation in the image centroid M_1_(r^bar^) as a function of radius is evidence for a nonequilibrium feature in the intracluster medium (ICM) density distribution. In simple terms, centroid shifts indicate that the center of mass of the ICM varies with radius. This variation is a tracer of continuing dynamical evolution. For each cluster, we evaluate the significance of variations in the centroid of the IPC image by computing the same statistics on an ensemble of simulated cluster images. In producing 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 (A426 [Perseus], A2256, A3526 [Centaurus], A1656 [Coma], and A3266 [Sersic 40/6]) reveals that the core of each one has significant substructure. In addition, we find significant variations in the orientation angle {theta}_2_(r^bar^) and the axial ratio n(r^bar^) for several of the clusters. Subject headings: galaxies: clustering - X-rays: galaxies
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