Thermal structure of gas in relaxed clusters of galaxies
  Gas clouds under the influence of gravitation in thermodynamic equilibrium cannot be isothermal due to the Dufour effect, the energy flux induced by density gradients. In galaxy clusters this effect may be responsible for most of the "cooling flows" instead of radiative cooling of the gas. Observations from XMM-Newton and Chandra satellites agree well with model calculations, assuming equilibrium between energy flux induced by temperature and density gradients, but without radiation loss. 
E. Fischer