dark matter (DM) potential wells (hence the luminosity) of the galaxies hosting them. Indeed, while hierarchical models predict massive objects to be assembled at later cosmic times, they also predict their progenitor clumps to be formed in biased-high density regions of the primordial density field, where the enhanced density allowed early star formation.

Thus, the abundance of red, massive galaxies resulting from such models results from the balance between the later epoch of assembly predicted for the most massive objects and the older stellar population predicted to be in place in the progenitors of such objects. Such a delicate balance makes the comparison with the observations concerning the abundance and the color distributoin of massive galaxies a sensible probe for such models.

Indeed, recent models, including the enhancement in the star formation of progenitors of massive galaxies triggered by interactions, are able to match many global properties of the evolving galaxy population, like the observed decline of the global stellar mass density from $z=0$ to $z=2$ (see, e.g., Fontana et al. 2004) and the evolution of the B and UV luminosity functions (Somerville , Primack & Faber 2001; Menci et al. 2004a; Dahlen et al. 2005) from $z=0$ to $z=4$. 