  Recent measurements of the K-band luminosity function now provide us with
strong, reliable constraints on the fraction of baryons which have cooled.
Globally, this fraction is only about 5%, and there is no strong evidence that
it is significantly higher in clusters. Without an effective sub-grid feedback
prescription, the cooled gas fraction in any numerical simulation exceeds these
observational constraints, and increases with increasing resolution. This
compromises any discussion of galaxy and cluster properties based on results of
simulations which include cooling but do not implement an effective feedback
mechanism.
