The Cooling Flow to Accretion Flow Transition

Cooling flows in galaxy clusters and isolated elliptical galaxies are asource of mass for fueling accretion onto a central supermassive black hole. Wecalculate the dynamics of accreting matter in the combined gravitationalpotential of a host galaxy and a central black hole assuming a steady state,spherically symmetric flow (i.e., no angular momentum). The global dynamicsdepends primarily on the accretion rate. For large accretion rates, no simple,smooth transition between a cooling flow and an accretion flow is possible; thegas cools towards zero temperature just inside its sonic radius, which lieswell outside the region where the gravitational influence of the central blackhole is important. For accretion rates below a critical value, however, theaccreting gas evolves smoothly from a radiatively driven cooling flow at largeradii to a nearly adiabatic (Bondi) flow at small radii. We argue that this isthe relevant parameter regime for most observed cooling flows. The transitionfrom the cooling flow to the accretion flow should be observable in M87 withthe {\it Chandra X-ray Observatory}.Comment: emulateapj.sty, 10 pages incl. 5 figures, to appear in Ap