Downsizing of star-forming galaxies by gravitational processes

Context. There is observed a trend that a lower mass galaxy forms stars at a later epoch. This downsizing of star-forming galaxies has been attributed to hydrodynamical or radiative feedback processes that regulate star formation. Aims. We explain the downsizing by gravitational processes alone, in the bottom-up scenario where galaxies evolve from subgalactic-scale objects. Methods. Using the theory of Press and Schechter, as a function of galaxy mass, we study the peak epoch of formation of subgalactic-scale objects, i.e., gravitational collapse of subgalactic-scale density fluctuation. Results. The subgalactic-scale density fluctuation is found to collapse at a later epoch for a lower mass galaxy. The epoch is close to the peak epoch of star formation derived from observations. Conclusions. The downsizing is inherent in gravitational processes of the bottom-up scenario. 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. Gravitational collapse of the subgalactic-scale density fluctuation and the subsequent onset of star formation occur at a later epoch for a lower mass galaxy.