The bright end of the colour-magnitude relation of cluster galaxies
  We investigate the development of the red sequence (RS) of cluster galaxies
by using a semi-analytic model of galaxy formation. Results show good agreement
between the general trend of the simulated RS and the observed relation in
early-type galaxies. However, the most luminous galaxies ($M_V \lesssim -20$)
depart from the linear fit to observed data, displaying almost constant
colours. We analyze the dependence with redshift of the fraction of stellar
mass contributed to each galaxy by different processes (i.e., quiescent star
formation, disc instability and mergers), finding that the evolution of the
bright end, since $z\approx 2$, is mainly driven by minor and major dry
mergers. Since the most luminous galaxies have a narrow spread in ages
($1.0\times 10^{10}$ yr $<t<1.2\times 10^{10}$ yr), their metallicities are the
main factor that affects their colours. Galaxies in the bright end reach an
upper limit in metallicity as a result of the competition of the mass of stars
and metals provided by the star formation within the galaxies and by the
accretion of merging satellites. Star formation activity in massive galaxies
(M_\star \gtrsim 10^{10} M_{\odot}$) contribute with stellar components of high
metallicity, but this fraction of stellar mass is negligible. Mergers
contribute with a larger fraction of stellar mass ($\approx 10-20$ per cent),
but the metallicity of the accreted satellites is lower by $\approx 0.2$ dex
than the mean metallicity of galaxies they merge with. The effect of dry
mergers is to increase the mass of galaxies in the bright end, without
significantly altering their metallicities, and hence,their colours, giving
rise to the break in the RS. These results are found for clusters with
different virial masses, supporting the idea of the universality of the CMR in
agreement with observational results.
