What galaxies know about their nearest cluster
  We investigate the extent to which galaxies' star-formation histories and morphologies are determined by their clustocentric distance and their nearest cluster's richness. We look at three tracers of star-formation history (i-band absolute magnitude M_i, [g-r] color, and Halpha emission line equivalent width) and two indicators of galaxy morphology (surface brightness and radial concentration) for 52,569 galaxies in the redshift range of 0.015<z<0.068. We find that our morphology indicators relate to the clustocentric distance only indirectly through their relationships with stellar population and starformation rate. Galaxies that are near the cluster center tend to be more luminous, redder and have lower Halpha EW (ie, lower star-formation rates) than those that lie near or outside the virial radius of the cluster. The detailed relationships between these galaxy properties and clustocentric distance depend on cluster richness. For richer clusters, we find that (i) the transition in color and Halpha EW from cluster center to field values is more abrupt and occurs closer to the cluster virial radius, and (ii) the color and Halpha EW distributions are overall narrower than in less rich clusters. We also find that the radial gradient seen in the luminosity distribution is strongest around the smaller clusters and decreases as cluster richness increases. We find there is a `characteristic distance' at around one virial radius (the infall region) where the change with radius of galaxy property distributions is most dramatic, but we find no evidence for infall-triggered star bursts. These results suggest that galaxies `know' the distance to, and the size of, their nearest cluster and they express this information in their star-formation histories. 
Alejandro D. Quintero, Andreas A. Berlind, Michael R. Blanton, David   W. Hogg