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Transformation of Galaxy Morphology and Luminosity Classes

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 (ρ1). 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 ρ1 > ρvirial, its morphology depends also on the neighbor’s morphology and the large-scale background density over a few Mpc scales (ρ20) in addition to luminosity and ρ1. The early type probability keeps increasing as ρ1 increases if its neighbor is an early type. But the probability decreases as ρ1 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 ρ20 increases when ρ1 > ρvirial. 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 ρ1, 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 occur through distant interactions and mergers, which results in the morphology–luminosity–local density relation. Subject headings: galaxies:general – galaxies:formation – galaxies:evolution – galaxies:morphology – galaxies:luminosity