The Environmental Dependence of Galaxy Properties in the Local Universe: Dependences on Luminosity, Local Density, and System Richness

We investigate the environmental dependence of star formation and the morphology of galaxies in the local universe based on a volume-limited sample constructed from the data of the Sloan Digital Sky Survey. The sample galaxies (19,714 in total) are restricted to the redshift range of 0.030<z<0.065 and the magnitude range of M_r<M^*_r+2. We investigate correlations between star formation, morphology, luminosity, local density, and richness of galaxy systems. First, we focus on how galaxy properties change with local density. Galaxies in a dense environment are found to have suppressed star formation rates and early morphological types compared with those in the field. Star formation and morphology show a ``break'' at the critical local density of logΣ_crit~0.4galaxiesh²₇₅Mpc^-2, which is in agreement with previous studies. However, the break can be seen only for faint galaxies (M^*_r+1<M_r<M^*_r+2), and bright galaxies (M_r<M^*_r+1) show no break. Thus, galaxies of different luminosities are found to show different environmental dependences. Next we examine dependences on richness of galaxy systems. Median properties of galaxies residing in systems with σ>200kms^-1 show no dependence on system richness, and most of the galaxies in those systems are non-star-forming early-type galaxies. Star formation activities of galaxies are different from those of field galaxies even in systems as poor as σ~100kms^-1. This result suggests that environmental mechanisms that are effective only in rich systems, such as ram pressure stripping of cold gas and harassment, have not played a major role in transforming galaxies into red early-type galaxies. Strangulation and interactions between galaxies, however, remain candidates of the driver of the environmental dependence. In the dense environment in the local universe, the slow transformation of faint galaxies occurs to some extent, but the transformation of bright galaxies is not clearly visible. We suggest that the evolution of bright galaxies is not strongly related to galaxy system, such as groups and clusters, while the evolution of faint galaxies is likely to be closely connected to galaxy system.