Physics of the Merging Clusters Cygnus A, A3667, and A2065
  We present ASCA gas temperature maps of the nearby merging galaxy clusters Cygnus A, A3667, and A2065. Cygnus A appears to have a particularly simple merger geometry that allows an estimate of the subcluster collision velocity from the observed temperature variations. We estimate it to be ~2000 km/s. Interestingly, this is similar to the free-fall velocity that the two Cygnus A subclusters should have achieved at the observed separation, suggesting that merger has been effective in dissipating the kinetic energy of gas halos into thermal energy, without channeling its major fraction elsewhere (e.g., into turbulence). In A3667, we may be observing a spatial lag between the shock front seen in the X-ray image and the corresponding rise of the electron temperature. A lag of the order of hundreds of kiloparsecs is possible due to the combination of thermal conduction and a finite electron-ion equilibration time. Forthcoming better spatial resolution data will allow a direct measurement of these phenomena using such lags. A2065 has gas density peaks coincident with two central galaxies. A merger with the collision velocity estimated from the temperature map should have swept away such peaks if the subcluster total mass distributions had flat cores in the centers. The fact that the peaks have survived (or quickly reemerged) suggests that the gravitational potential also is strongly peaked. Finally, the observed specific entropy variations in A3667 and Cygnus A indicate that energy injection from a single major merger may be of the order of the full thermal energy of the gas. We hope that these order of magnitude estimates will encourage further work on hydrodynamic simulations, as well as more quantitative representation of the simulation results. 
M. Markevitch (CfA), C. L. Sarazin (UVA), and A. Vikhlinin (CfA)