  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.
