The Environmental Dependence of Galaxy Properties in the Local Universe:   Dependence 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 (0.030<z<0.065 and M_r<M_r^*+2) constructed from the data of the Sloan Digital Sky Survey. First, we focus on how galaxy properties change with local density. The star formation and morphology show a 'break' at the critical local density of log Sigma_crit~0.4 galaxies h_75^2 Mpc^-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 dependencies. Next, we examine dependencies on richness of galaxy systems. Star formation activities of galaxies are different from those of field galaxies even in systems as poor as sigma~100 km/s. 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 environments 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. 
Masayuki Tanaka, Tomotsugu Goto, Sadanori Okamura, Kazuhiro Shimasaku,   Jon Brinkman