Infant Galaxy Clusters at Low Redshifts?

The population and population composition of galaxies in galaxy clusters at present reflect the mass of the clusters and the mass growth of the galaxies in the past. We investigate them for six clusters. We show that galaxies in massive clusters stopped growing at redshifts of ~4. Moreover, we find that some small galaxy clusters (groups) have too many massive galaxies for their apparent masses. One possibility is that these groups are much more massive and in a phase just before virialization. If this is the case, they should be called `infant galaxy clusters' that will be matured clusters in the dynamical time-scale (~10^9 yrs).