X-ray measurements of the nonspherical mass distribution in the cluster of galaxies A2256

Abstract

The authors describe a technique for the analysis of X-ray surface brightness maps of rich clusters of galaxies with a smooth X-ray morphology and appropriate bilateral symmetry to yield an accurate estimate of the underlying cluster mass distribution. This method assumes that the X-ray emitting gas is in hydrostatic equilibrium in the gravitational potential of the cluster, and that the cluster possesses an axis of symmetry that lies in the plane of the sky. The errors associated with the latter assumption are discussed. The authors apply this technique to the cluster of galaxies A2256 and find that the mass distribution departs significantly from spherical symmetry, with axial ratios of approximately 1.6. This is to be compared with the axial ratio of 2 - 3.3 found for the galaxy distribution. They find that the surfaces of constant total density are more complex than similar oblate or prolate spheroids with a fixed center. The radially integrated mass derived from the X-ray data is consistent with that estimated from the galaxy surface density and velocity dispersion.