Entropy Limit and the Cold Feedback Mechanism in Cooling Flow Clusters
  I propose an explanation to the finding that star formation and visible filaments strong in Halpha emission in cooling flow clusters occur only if the minimum specific entropy and the radiative cooling time of the intracluster medium (ICM), are below a specific threshold. The explanation is based on the cold feedback mechanism. In this mechanism the mass accreted by the central black hole originates in non-linear over-dense blobs of gas residing in an extended region of the cooling flow region. I use the criterion that the feedback cycle period must be longer than the radiative cooling time of dense blobs for large quantities of gas to cool to low temperature. The falling time of the dense blobs is parameterized by the ratio of the infall velocity to the sound speed. Another parameter is the ratio of the blobs' density to that of the surrounding ICM. By taking the values of the parameters as in previous papers on the cold feedback model, I derive an expression that gives the right value of the entropy threshold. Future studies will have to examine in more detail the role of these parameters, and to show that the observed sharp change in the behavior of clusters across the entropy, or radiative cooling time, threshold can be reproduced by the model. 
Noam Soker (Technion, Israel)