The cooling flow to accretion flow transition

Cooling flows in galaxy clusters and isolated elliptical galaxies are a sou rce of mass for fueling accretion onto a central supermassive black hole. W e calculate the dynamics of accreting matter in the combined gravitational potential of a host galaxy and a central black hole assuming a steady state , spherically symmetric how (i.e., no angular momentum). The global dynamic s depends primarily on the accretion rate. For large accretion rates no sim ple, smooth transition between a cooling flow and an accretion flow is poss ible; the gas cools toward zero temperature just inside its sonic radius, w hich lies well outside the region where the gravitational influence of the central black hole is important. For accretion rates below a critical value , however, the accreting gas evolves smoothly from a radiatively driven coo ling flow at large radii to a nearly adiabatic (Bondi) flow at small radii. We argue that this is the relevant parameter regime for most observed cool ing flows. The transition from the cooling flow to the accretion flow shoul d be observable in M87 with the Chandra X-Ray Observatory.