Temperature Structure of Four Hot Clusters Obtained with [ITAL]ASCA[/ITAL] Data

ASCA data are used to obtain two-dimensional gas temperature maps of A2256,A2319, A2163 and A665. In all four, the temperature decreases significantly atoff-center distances of ~1 h^{-1} Mpc. Central regions of the two nearerclusters A2256 and A2319 appear largely isothermal except for the cooler spotscoincident with the subunits in their X-ray surface brightness. Although theexistence of this substructure may suggest ongoing merger activity, noasymmetric features in the temperature distribution resembling those in thehydrodynamic merger simulations are apparent. In the outer parts of theclusters, the temperature declines symmetrically with radius. In A2256 andA2319, it follows a polytropic slope with gamma~1.3-1.5. This is somewhatsteeper than the simulations predict for a flat CDM universe and is closer tothe open universe predictions (Evrard etal 1996). The temperature drop is moreprominent in the outer regions of A2163 and A665 and appears even steeper thanadiabatic (although not inconsistent with it). If the gas in the outskirts ofthese two clusters is indeed as cool as we measure, the cluster atmospheresshould be convectively unstable and transient. Also, such a steep temperatureprofile could not possibly emerge if the gas was heated only via the release ofits own gravitational energy during infall. This may indicate the presence ofan additional heat source in the inner cluster, such as merger shockstransferring energy from the dark matter to the gas.Comment: Latex, 8 pages + color plate, uses aaspp4.sty, flushrt.sty and pstricks.sty. Accepted for ApJ Letter