Solving the Cooling Flow Problem through Mechanical AGN Feedback
Unopposed radiative cooling of plasma would lead to the cooling catastrophe, a massive inflow of condensing gas, manifes t in the core of galaxies, groups and clusters. The last genera tion X-ray telescopes, Chandra and XMM, have radically changed our view on baryons, indicating AGN heating as the balancing counterpart of cooling. This work reviews our extensive investigation on self-regulated heating. We argue that the mechanical feedback, based on massive subrelativistic outflows, is the key to solving the cooling flow problem, i.e. d ramatically quenching the cooling rates for several Gyr without destroying the cool-core structure. Using a modifie d version of the 3D hydrocode FLASH, we show that bipolar AGN outflows can further reproduce fundamental observed fea tures, such as buoyant bubbles, weak shocks, metals dredgeup, and turbulence. The latter is an essential ingredient to drive nonlinear thermal instabilities, which cause the for mation of extended cold gas, a residual of the quenched cooling flow a nd, later, fuel for the feedback engine. Compared to clusters, groups and galaxies require a gentler mechanical feedback, in order to avoid catastrophic overheating. We highlight the essential characteristics for a realistic AGN feedback, wi th emphasis on observational consistency.