5 What galaxies know about their nearest cluster

We measure the clustocentric distances of 52,569 galaxies in the redshift range of 0.015 < z < 0.068. By “clustocentric distance” we mean the transverse projected distance to the center of the nearest cluster within ±2000 km s. By “cluster” we mean a bound group of at least Ngal galaxies brighter than Mr = −19 mag; we consider different “richness” criteria of Ngal = 5, 10, and 20. We examine the relationship between the clustocentric distances and three tracers of star-formation history (i band absolute magnitude M0.1i, [g−r] color, and Hα emission line equivalent width); and two indicators of galaxy morphology (surface brightness and radial concentration). We find that surface brightness and concentration relate to the clustocentric distance only indirectly through their relationships with stellar population and star formation rate. Galaxies that are near the cluster center tend to be more luminous, redder and have lower Hα EW (i.e., 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 Hα EW from cluster center to field values is more abrupt and occurs closer to the cluster virial radius, and (ii) the color and Hα 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 the cluster richness (and mass) 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. Subject headings: galaxies: clustering — galaxies: clusters: general — galaxies: evolution — galaxies: fundamental parameters — galaxies: statistics — galaxies: stellar content 1 Center for Cosmology and Particle Physics, Department of Physics, New York University, 4 Washington Place, New York, NY 10003 2 To whom correspondence should be addressed: [email protected]