Downsizing of Star-forming Galaxies Explained by Gravitational Processes

There is a trend that lower mass galaxies form stars at later epochs. This downsizing of star-forming galaxies has been attributed to hydrodynamical or radiative feedback processes that regulate star formation. However, here we explain the downsizing by gravitational processes alone, in the bottom-up scenario where galaxies evolve from subgalactic-scale objects. Within a region of the initial density field that is to evolve into a lower mass galaxy, subgalactic-scale fluctuation is of a smaller amplitude. The formation of subgalactic-scale objects, i.e., gravitational collapse of the subgalactic-scale fluctuation, and the subsequent onset of star formation accordingly occur at a later epoch for a lower mass galaxy. As a function of galaxy mass, we calculate the peak epoch of formation of subgalacticscale objects. The peak epoch is consistent with the peak epoch of star formation derived from observations. Subject headings: cosmology: theory — galaxies: evolution — galaxies: formation