A Two-Fluid Thermally-Stable Cooling Flow Model
  A new model for cooling flows in X-ray clusters, capable of naturally explaining salient features observed, is proposed. The only requirement is that a significant relativistic component, in the form of cosmic rays (CR), be present in the intra-cluster medium and significantly frozen to the thermal gas. Such an addition qualitatively alters the conventional isobaric thermal instability criterion such that a fluid parcel becomes thermally stable when its thermal pressure drops below a threshold fraction of its CR pressure. Consequently, the lowest possible temperature at any radius is about one third of the ambient temperature {\it at that radius}, exactly as observed, In addition, we suggest that dissipation of internal gravity waves, excited by radial oscillatory motions of inward drifting cooling clouds about their radial equilibrium positions, may be responsible for heating up cooling gas. With the ultimate energy source for powering the cooling X-ray luminosity and heating up cooling gas being gravitational due to inward drifting cooling clouds as well as the general inward flow, heating is spatially distributed and energetically matched with cooling. One desirable property of this heating mechanism is that heating energy is strongly centrally concentrated, providing the required heating for emission-line nebulae. 
Renyue Cen (Princeton University Observatory)