  There is abundant evidence that heating processes in the central regions of
elliptical galaxies has both prevented large-scale cooling flows and assisted
in the expulsion of metal rich gas. We now know that each such spheroidal
system harbors in its core a massive black hole weighing approximately 0.13% of
the mass in stars and also know that energy was emitted by each of these black
holes with an efficiency exceeding 10% of its rest mass. Since, if only 0.5% of
that radiant energy were intercepted by the ambient gas, its thermal state
would be drastically altered, it is worth examining in detail the interaction
between the out-flowing radiation and the equilibrium or inflowing gas. On the
basis of detailed hydrodynamic computations we find that relaxation
oscillations are to be expected with the radiative feedback quite capable of
regulating both the growth of the central black hole and also the density and
thermal state of the gas in the galaxy. Mechanical input of energy by jets may
assist or dominate over these radiative effects. We propose specific
observational tests to identify systems which have experienced strong bursts of
radiative heating from their central black holes.
