  In conventional models of galactic and cluster cooling flows widespread
cooling (mass dropout) is assumed to avoid accumulation of unacceptably large
central masses. However, recent XMM observations have failed to find spectral
evidence for locally cooling gas. This has revived the notion that cooling
flows are heated by some process such as an intermittent, low-level AGN
involving supermassive black holes in the central galaxy. To explore this
hypothesis, we consider the gasdynamical consequences of galactic cooling flows
heated by many different scenarios without specifying the detailed physics of
the heating process. We are unable to find a single acceptable heated flow in
reasonable agreement with well observed hot gas temperature and density
profiles, even using finely tuned parameters. Idealized flows in which
radiative cooling is perfectly balanced by global heating are grossly
incompatible with observations. Flows heated by episodic central feedback
generate quasi-cyclic changes in the hot gas density profile which are not
supported by current observations. Paradoxically, centrally heated (or
pressurized) cooling flows experience spontaneous non-linear compressions that
result in spatially widespread cooling instabilities. Therefore, spectral
evidence for cooling gas is difficult to avoid by central heating.
