We investigate the color-magnitude diagram (CMD) of cluster galaxies in the
hierarchical Λ-CDM cosmological scenario using both single stellar
populations and simple galaxy models. First, we analyze the effect of bursts
and mergers and companion chemical pollution and rejuvenation of the stellar
content on the integrated light emitted by galaxies. The dispersion of the
galaxy magnitudes and colors on the MV​−(B−V) plane is mainly due to mixing
of ages and metallicities of the stellar populations, with mergers weighting
more than bursts of similar mass fractions. The analysis is made using the
Monte-Carlo technique applied to ideal model galaxies reduced to single stellar
populations with galaxy-size mass to evaluate mass, age and metallicity of each
object. We show that separately determining the contributions by bursts and
mergers leads to a better understanding of observed properties of CMD of
cluster galaxies. Then we repeat the analysis using suitable chemo-photometric
models of galaxies whose mass is derived from the cosmological predictions of
the galaxy content of typical clusters. Using the halo mass function and the
Monte-Carlo technique, we derive the formation redshift of each galaxy and its
photometric history. These are used to simulate the CMD of the cluster
galaxies. The main conclusion is that most massive galaxies have acquired the
red color they show today in very early epochs and remained the same ever
since. The simulations nicely reproduce the Red Sequence, the Green Valley and
the Blue Cloud, the three main regions of the CMD in which galaxies crowd.Comment: Accepted for publication in Ap
