CLUES TO GALAXY ACTIVITY FROM RICH CLUSTER SIMULATIONS

New simulations of rich cluster evolution are used to evaluate the 'first infall' hypothesis of the Butcher-Oemler effect - the idea that the enhanced fraction of active galaxies seen in high-redshift clusters is due to a one-time burst of star formation triggered by the rapid rise in external pressure as a galaxy ploughs into the hot intracluster medium (ICM). Using three-dimensional simulations which contain both baryonic gas and collisionless dark material, local static pressure histories for test orbits of galaxies are generated and a simple trigger threshold based on dP/dt\P(ISM) is applied to define an 'active' fraction of the population. The results lend firm qualitative and some quantitative support to the first infall interpretation.