Velocity Dispersion of 335 Galaxy Clusters Selected from the Sloan Digital Sky Survey: Statistical Evidence for Dynamical Interaction, and Against Ram-Pressure Stripping

There has been plenty of observational evidence of cluster galaxy evolution such as the Butcher-Oemler effect and the morphology-density relation. However, it has been difficult to identify the origin of the cluster galaxy evolution. It has been simply difficult to trace the complicated process of galaxy evolution with several giga years of timescale, using the observation which only brings us with information of a single epoch. Here we show that gravitational interaction/friction between galaxies is the statistically dominant physical mechanism responsible for the cluster galaxy evolution, and that the well-favored ram-pressure stripping by the cluster gas is not statistically driving the cluster galaxy evolution. We have constructed the largest composite cluster with 14548 member galaxies out of 335 clusters with σ > 300 km s carefully selected from the Sloan Digital Sky Survey. By measuring velocity dispersions of various subsamples of galaxies in this composite cluster, we found that bright cluster galaxies (Mz < −23) have significantly smaller velocity dispersion than faint galaxies (Mz > −23), with much greater precision than previous results (e.g., Adami et al.). We interpret this as direct evidence of the dynamical interaction/friction between cluster galaxies, where massive galaxies lose their velocity through the energy equipartition during the dynamical interaction/friction with less massive galaxies. We also found that star-forming late-type galaxies have a larger velocity dispersion than passive late-type galaxies. This is inconsistent with the ram-pressure stripping model; since the ram-pressure is proportional to σv (i.e., stronger for galaxies with high velocity), the ram-pressure stripping cannot explain the observed trends of passive (evolved) galaxies having low velocity rather than high velocity. On the other hand, the result is again consistent with the dynamical galaxy-galaxy interaction/friction, where more evolved (passive) galaxies lose their velocity through dynamical interaction/friction.