Diffuse X-Rays as a Probe to Survey the Gravitational Mass Distribution in the Coma Cluster of Galaxies

Abstract Diffuse X-rays from the hot plasma in a cluster of galaxies are investigated by assuming its hydrostatic equilibrium in the gravitational potential of the cluster. By reproducing the observed X-ray brightness profile of the Coma cluster of galaxies, we determine its total gravitational mass as (2–3)×1015M◉ and its core radius as about 10′, 1.5 times as large as the conventional scale for the distribution of the bright member galaxies. The local enhancement of X-ray emission is calculated by taking into account the potential wells of the two cD galaxies, NGC 4874 and 4889, at the cluster center.

Comparing the calculated X-ray maps with the detailed X-ray distribution obtained by the Einstein Observatory, we conclude that each mass of these supergiant galaxies is less than 1×1013M◉ corresponding to the mass-to-luminosity ratio less than 50M◉/L◉, or that a mere fraction of the gravitational mass of the cluster is concentrated in the visible member galaxies. Otherwise the hot plasma in the deep potential wells will emit many more X-rays locally than are observed by the high-resolution detector on board the Einstein Observatory.