  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.
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 subgalactic-scale
objects. The peak epoch is consistent with the peak epoch of star formation
derived from observations. [abridged]
