Temperature Structure of Four Hot Clusters with ASCA
  ASCA data are used to obtain two-dimensional gas temperature maps of A2256, A2319, A2163 and A665. In all four, the temperature decreases significantly at off-center distances of ~1 h^{-1} Mpc. Central regions of the two nearer clusters A2256 and A2319 appear largely isothermal except for the cooler spots coincident with the subunits in their X-ray surface brightness. Although the existence of this substructure may suggest ongoing merger activity, no asymmetric features in the temperature distribution resembling those in the hydrodynamic merger simulations are apparent. In the outer parts of the clusters, the temperature declines symmetrically with radius. In A2256 and A2319, it follows a polytropic slope with gamma~1.3-1.5. This is somewhat steeper than the simulations predict for a flat CDM universe and is closer to the open universe predictions (Evrard etal 1996). The temperature drop is more prominent in the outer regions of A2163 and A665 and appears even steeper than adiabatic (although not inconsistent with it). If the gas in the outskirts of these two clusters is indeed as cool as we measure, the cluster atmospheres should be convectively unstable and transient. Also, such a steep temperature profile could not possibly emerge if the gas was heated only via the release of its own gravitational energy during infall. This may indicate the presence of an additional heat source in the inner cluster, such as merger shocks transferring energy from the dark matter to the gas. 
Maxim Markevitch (U. of Virginia)