Radial alignment of cluster galaxies

We report the discovery of a statistically significant radial alignment of cluster galaxies in a sample of 85 X-ray-selected clusters observed in the Sloan Digital Sky Survey. The tendency for alignment is a robust feature of our sample and does not vary significantly with individual cluster or galaxy properties. We use dynamical arguments to show that a significant fraction of cluster galaxies should be undergoing a parametric tidal resonance that can cause their long axes to orient themselves toward the center of the cluster potential, and therefore tentatively ascribe the observed radial alignment to this dynamical effect.