A moderate cluster cooling flow model

We propose that the outer portions of cooling flows in clusters of galaxies are frequently disrupted by radio jets and that their effective ages are m uch shorter than the cluster ages. The inner regions, where the gas density is higher, are more difficult to disrupt and may continue to harbor coolin g flows even after disruption events. The main assumption of the proposed s cenario is that, on a timescale of similar to2-4 x 10(9) yr, the cD galaxie s in cooling flow clusters undergo powerful bursts of active galactic nucle us (AGN) activity that produce strong radio jets. The radio jets excite sho cks in the inner regions (r less than or similar to 100 kpc) of cooling flo w clusters. A radio burst may result from the accretion of cooling material by the central black hole or from a collision with a subcluster. We assume that the jets remain strong, with kinetic powers of similar to 10(47) ergs s(-1) for similar to 10(7) yr. The jets excite shock waves moving at sever al times 10(3) km s(-1) and heat the cooling flow region, hence terminating it in the outer regions. The proposed scenario predicts that the total acc reted mass due to the cooling flow is an order of magnitude lower than the mass accreted according to the "standard" cooling flow model (which assumes an undisturbed cooling flow for a time equal to the age of the cluster). T he scenario, therefore, brings into agreement the observations that (1) a l arge fraction of clusters harbor cooling flows, (2) strong optical and radi o activity are present only in the very inner regions of cooling flows, and (3) there is a lack of a satisfactory reservoir of the expected (in the st andard cooling flow model) large mass that has been cooling over the life o f the cluster.