At higher redshift the problem is even more severe: the abundance of extremely red objects (EROs, with optical infra-red colors redder than a passively evolving elliptical, $R-K>5$) at $z\approx 1-2.5$ is underestimated by present hierarchical models (see McCarthy et al 2004; Cimatti et al. 2004; Daddi et al. 2005; Somerville et al. 2004) by factors up to ten (depending on the exact color and magnitude cut). This suggests that an additional process must be at work in determining the observed properties of galaxies; the overall agreement of hierarchical model predictions with the global (not color-selected) evolutionary properties of galaxies discussed above suggests that such a process should not constitute the main driver, but rather a complementary physical mechanism affecting mainly the evolution of the bright galaxy population.

In this respect, a possible solution has been proposed in terms of the feedback from Active Galactive Nuclei (AGNs) hosted at the center of galaxies (see, e.g., Ciotti & Ostriker 1997; Silk & Rees 1998, Haehnelt, Natarajan & Rees 1998, Fabian 1999). Indeed, the role of the energy injection from the AGNs into the interstellar medium of the host galaxies is at present one of the most pressing issues in the study of 