The observed properties of high-redshift cluster galaxies

We use the semi-analytic models of galaxy formation developed by Kauffmann,White \& Guiderdoni to generate predictions for the observed properties ofcluster and group galaxies at redshifts between 0 and 0.6. We examine four setsof cosmological initial conditions: low-density CDM models with and withoutcosmological constant, a flat CDM model and a mixed dark matter model. Thesemodels were selected because they span a wide range in cluster formation epoch.The semi-analytic models that we employ are able to follow both the evolutionof the dark matter component of clusters and the formation and evolution of thestellar populations of the cluster galaxies. We are thus able to generate modelpredictions that can be compared directly with the observational data. In thelow-density CDM models, clusters form at high red- shift and accrete verylittle mass at recent times. Our models predict that essentially no evolutionin the observed properties of clusters will have occurred by a redshift of 0.6,in direct contradiction with the data. In contrast, in the MDM model, bothgalaxies and clusters form extremely late. This model predicts evolution whichappears to be too extreme to be in agreement with the observations. The flatCDM model, which is intermediate in structure formation epoch, is mostsuccessful. This model is able to account for the evolution of the bluefraction of rich clusters with redshift, the relationship between blue fractionand cluster richness at different epochs, and the changes in the distributionof the morphologies of cluster galaxies by a redshift of 0.4.Comment: Latex file, 12 pages, postscript figures on request, 99