  We address the origin of the `downsizing' of elliptical galaxies, according
to which the stars in more massive galaxies formed earlier and over a shorter
period than those in less massive galaxies. We show that this could be the
natural result of a shutdown of star formation in dark matter haloes above a
critical mass of 10^12MSun. This is demonstrated using a semianalytic
simulation of galaxy formation within the standard hierarchical scenario of
structure formation. The assumed threshold mass is motivated by the prediction
of stable shock heating above this mass and the finding that such a shutdown
reproduces the observed distribution of galaxies in luminosity and colour.The
shutdown at a critical halo mass introduces a characteristic stellar mass for
the transition of galaxies into the `red sequence' of the galaxy
colour-magnitude diagram. Central galaxies of haloes that are more massive
today have reached this mass earlier and can therefore grow further along the
red sequence by dry mergers, ending up more massive and containing older stars.
Small galaxies formed in haloes below the critical mass can shutdown late, when
they fall into haloes above the critical mass and become satellites. While our
semianalytic simulation that incorporates an explicit shutdown reproduces
downsizing as inferred from the stellar ages of ellipticals, we explain why it
is much harder to detect downsizing using the mass functions of different
galaxy types.
