Pre-Processing of Galaxies before Entering a Cluster

We consider several mechanisms that possibly affect the evolution of disk galaxies in clusters using analytical models based on a hierarchical clustering scenario. We especially focus on the evolution of disk galaxies in subclusters located around a main cluster. We show that ram-pressure stripping cannot be always ignored in subclusters, although their masses are much smaller than that of the main cluster. The star-formation rate of a galaxy may gradually decrease by the stripping of warm gas (‘strangulation’) in a main cluster. However, we find that ram-pressure stripping could start before the strangulation is completed, if a field galaxy directly falls into the main cluster. Since this conflicts with some recent observations, many galaxies might have been affected by some environmental effects when they were in subclusters before they fell into the main cluster (‘pre-processing’). We show that strangulation and evaporation of the cold gas by the surrounding hot ICM in subclusters are the possible candidates. We also show that the observed morphological transformation of disk galaxies at z < ∼ 1 is not chiefly due to galaxy mergers.