Isothermality of the Gas in the Coma Cluster

We analyze the spectral and spatial X-ray data for the Coma cluster using nonisothermal temperature distributions. The spectrum was obtained by the Japanese X-ray satellite Tenma and the spatial data came from a reanalysis of the Einstein Observatory images. The superior energy response of the Tenma instrument allows us to determine the iron abundance more accurately than before; a value of between 16% and 32% (independent of model), relative to the cosmic value, is indicated. We find that the X-ray data alone can be described adequately by either an isothermal or an adiabatic gas temperature distribution. However, the cluster gravitational potentials inferred for the two models are quite different. If the temperature distribution is isothermal (or nearly so), then the cluster binding mass increases as r^1^ for large radii, and we derive a binding mass of ~1.5 x 10^15^ M_sun_ within 3 Mpc (H = 50 km s^-1^ Mpc^-1^). If the gas temperature is adiabatic, then the binding mass must be considerably more centrally condensed than the galaxies or the gas; over 90% of the cluster mass is contained within 1 Mpc. Nevertheless, this solution is not self-consistent since the total binding mass density drops below the inferred gas density at ~1 Mpc. Neither of these models is entirely satisfactory from our point of view, and so, based on theoretical considerations of thermal conduction in the intracluster medium, we propose a hybrid model consisting of a central isothermal region surrounded by a polytropic distribution. We determine limits on the size of the isothermal region as a function of the central, isothermal temperature, and we find that as much as 75% of the global emission can be coming from the isothermal component.