Transformation of morphology and luminosity classes of the SDSS galaxies

We present a unified picture on the evolution of galaxy luminosity and morphology. Galaxy morphology is found to depend critically on the local environment set up by the nearest neighbor galaxy in addition to luminosity and the large-scale density. When a galaxy is located farther than the virial radius from its closest neighbor, the probability for the galaxy to have an early morphological type is an increasing function only of luminosity and the local density due to the nearest neighbor (rho(n)). The tide produced by the nearest neighbor is thought to be responsible for the morphology transformation toward the early type at these separations. When the separation is less than the virial radius, i. e., when rho(n) > rho virial, its morphology depends also on the neighbor's morphology and the large-scale background density over a few megaparsec scales (rho(20)) in addition to luminosity and rho(n). The early-type probability keeps increasing as rho(n) increases if its neighbor is an early morphological type galaxy. But the probability decreases as rho(n) increases when the neighbor is a late type. The cold gas streaming from the late-type neighbor can be the reason for the morphology transformation toward late type. The overall early- type fraction increases as rho(20) increases when rho(n) > rho(virial). This can be attributed to the hot halo gas of the neighbor, which is confined by the pressure of the ambient medium held by the background mass. We have also found that galaxy luminosity depends on rho(n), and that the isolated bright galaxies are more likely to be recent merger products. We propose a scenario that a series of morphology and luminosity transformation occurs through distant interactions and mergers, which results in the morphology-luminosity-local density relation.