Environmental Dependence of Properties of Galaxies in the Sloan Digital Sky Survey

We investigate the dependence of physical properties of galaxies brighter than Mr = -18.0 + 5 log h in the Sloan Digital Sky Survey (SDSS) on environment, as measured by local density using an adaptive smoothing kernel. We find that variations of galaxy properties with environment are almost entirely due to the dependence of morphology and luminosity on environment. Because galaxy properties depend not only on luminosity but also on morphology, it is clear that galaxy properties cannot be determined solely by dark halo mass. When morphology and luminosity are fixed, other physical properties, such as color, color gradient, concentration, size, velocity dispersion, and star formation rate, are nearly independent of local density, without any break or feature. The only feature is the sharp decrease of the late-type fraction above the critical luminosity of about Mr = -21.3 in the morphology versus luminosity relation. Weak residual dependences on environment include that of the color of late types (bluer at lower density) and of the L-σ relation of early types (larger dispersion at higher density for bright galaxies). The fraction of galaxies with early morphological type is a monotonically increasing function of local density and luminosity. The morphology-density-luminosity relation, as measured in this work, should be a key constraint on galaxy formation models. We demonstrate that the dependence on environment of the morphology of galaxies originates from variations in density on effective Gaussian smoothing scales much smaller than 12 h-1 Mpc. We find that galaxy morphology varies both with density measured on an effective Gaussian smoothing scale of 4.7 h-1 Mpc and with distance to the nearest bright galaxy, particularly when the distance is about 0.2 h-1 Mpc. We propose as a mechanism that the morphology of galaxies in galaxy systems is transformed by the tidal force.