2002ApJ...573..542B

In conventional models of galactic and cluster cooling flows widespread cooling (mass dropout) is assumed to avoid accumulation of unacceptably large central masses. This assumption is often criticized because the observed mass of cooled gas and newly formed stars is much less than the cooling rates imply. Moreover, recent XMM observations have failed to find spectral evidence for locally cooling gas. This remarkable discovery has revived the notion that cooling flows do not cool in the manner previously assumed but are instead heated by some process such as an intermittent or low-level active galactic nucleus in the central galaxy. To explore this hypothesis, we consider the gasdynamical consequences of heated galactic cooling flows under many different heating scenarios without specifying the detailed physics of the heating process. We have been unable to find a single acceptable heated flow in reasonable agreement with well-observed hot gas temperature and density profiles. Even for poor fits to the observations, these models require finely tuned heating scenarios. Idealized flows in which radiative cooling is perfectly balanced by global heating are grossly incompatible with observations. Flows heated by central feedback often result in episodic cooling that is associated with quasi-cyclic changes in the hot gas density profile; at present there is no observational evidence for this. Paradoxically, when cooling flows are centrally heated (or pressurized), they experience spontaneous nonlinear compressions that result in spatially widespread cooling instabilities. Therefore, spectral evidence for cooling gas is expected in either traditional cooling flows with ad hoc mass dropout or heated cooling flows without the dropout assumption.