The environmental dependence of galaxy properties in the local universe: effects of local and global environment

We investigate the environmental dependence of galaxy properties in the local universe based on the data of the Sloan Digital Sky Survey. We focus on how star formation and morphology of galaxies correlate with luminosity, local environment, and global environment. We find that galaxy properties abruptly change at a critical local density of log σ_crit∼ 0.4 galaxies h₇₅² Mpc^-2. The "break" at the critical density is found only for faint galaxies (M^*_r+1 < M_r < M^*_r+2). Bright galaxies (M_r < M^*_r+1) show no break. That is, the star formation-density and the morphology-density relations depend on galaxy luminosity. Next, we focus on global environment, i.e., richness of galaxy groups and clusters. Most galaxies are not forming stars in groups as poor as σ∼200 km s^-1. This fact suggests that environmental mechanisms that are effective only in rich clusters, such as ram-pressure stripping of cold gas and harassment, have not played a major role in suppressing galaxy star formation. Our results may suggest that evolution of bright galaxies is not strongly related to galaxy systems such as groups and clusters. On the other hand, evolution of faint galaxies may have a close connection.