Toward understanding environmental effects in SDSS clusters

We find groups and clusters of galaxies using the Data Releases DR1 and DR3 of the Sloan Digital Sky Survey. We calculate a low-resolution density field with a smoothing length of 10 h(-1) Mpc to characterise the density of the cluster environment, and a medium-resolution density field with a smoothing length of 2 h(-1) Mpc to characterise the galaxy environment. We determine the luminosity function of clusters, and investigate properties of galaxies and clusters in various environments. We show that clusters in a high-density environment are about 5 times more luminous than in a low-density environment, and luminosities of galaxies in different environments differ by a factor of similar to 25. We see similar effects in numerical simulations - simulated clusters in a high-density environment are similar to 100 times more massive than those in a low-density environment. Comparison of the density distribution in simulations at various epochs shows that in large low-density regions ( voids) dynamical evolution is very slow and stops early. In contrast, in large regions of higher density (superclusters) dynamical evolution starts early and continues until the present; here particles cluster early, and by merging of smaller groups very rich systems of galaxies form.