  We examined effects of the ultraviolet background radiation (UVB) on the
colour--magnitude relation (CMR) of elliptical galaxies in clusters of galaxies
in the hierarchical clustering scenario by using a semi-analytic model of
galaxy formation. In our model the UVB photoionizes gas in dark haloes and
suppresses the cooling of the diffuse hot gas onto galaxy discs. By using a
semi-analytic model without the effect of the UVB, Kauffmann & Charlot found
that the CMR can be reproduced by strong supernova heating because such
supernova feedback suppresses the chemical enrichment in galaxies especially
for small galaxies. We find that the CMR also becomes bluer because of the UVB,
in a different way from the effect of supernova feedback. While the supernova
feedback suppresses the chemical enrichment by a similar mechanism to galactic
wind, the UVB suppresses the cooling of the hot gas. This fact induces the
suppression of the metallicity of the intracluster medium (ICM). In our model
we find that the existence of the UVB can plausibly account for an observed ICM
metallicity that is equal to nearly 0.3 times the solar value, and that in this
case we can reproduce the CMR and the metallicity of the ICM simultaneously.
