In this thesis I present the results obtained during my PhD studies at SISSA in the field of galaxy formation, with focus on the problem of formation of Early Type Galaxies (ETGs) at high redshift. The interest to this issue has been increasing with time, since in the last decade observational evidence brought the idea that the formation and evolution
of quasars is strictly related to that of ETGs. In order to tackle the galaxy formation most of the efforts has been put in developing semi-analytical and numerical codes based on the assumption that the hierarchy of galaxy Dark Matter halos traces the history also of baryons (stars and gas) through continuous merging. On the other hand, the group in SISSA and in Padua Observatory put forward the concept that the galaxy and
quasar phenomenology at high redshift is related chiefly to physical processes (such as cooling, collapse, star formation, accretion onto the central Black Hole seed and feedback) occurring in the gas associated to the phase of fast collapse/merging of the central 30-40% of the mass of DM halos, while the later phase of slow accretion is little affecting the
evolution of stars and quasars.
