A Moderate Cluster Cooling Flow Model

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Abstract
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 much shorter than the cluster ages. The inner regions, where the gas density is higher, are more difficult to disrupt and may continue to harbor cooling flows even after disruption events. The main assumption of the proposed scenario is that, on a timescale of ~2-4 × 109 yr, the cD galaxies in cooling flow clusters undergo powerful bursts of active galactic nucleus (AGN) activity that produce strong radio jets. The radio jets excite shocks in the inner regions (r 100 kpc) of cooling flow 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 ~1047 ergs s-1 for ~107 yr. The jets excite shock waves moving at several times 103 km s-1 and heat the cooling flow region, hence terminating it in the outer regions. The proposed scenario predicts that the total accreted 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). The scenario, therefore, brings into agreement the observations that (1) a large fraction of clusters harbor cooling flows, (2) strong optical and radio 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 standard cooling flow model) large mass that has been cooling over the life of the cluster.
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