  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.
