The elliptical galaxy colour–magnitude relation as a discriminant between the monolithic and merger paradigms

The colour-magnitude relation (CMR) of cluster elliptical galaxies has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic collapse and merger paradigms of elliptical galaxy formation. We use a �CDM hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the star formation history (SFH) of cluster ellipticals predicted by the model is quasimonolithic, with only � 10 percent of the total stellar mass forming after z � 1. The quasi-monolithic SFH results in a predicted CMR that agrees well with its observed counterpart in the redshift range 0 < z < 1.27. We use our analysis to argue that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.