We investigate the evolutionary properties of a sample of quasars at 5 < z < 6.4 using the semi-analytical hierarchical model GAMETE/QSOdust .
We find that the observed properties of these quasars are well reproduced by a common formation scenario in which stars form according to a standard IMF , via quiescent star formation and efficient merger-driven bursts , while the central BH grows via gas accretion and BH-BH mergers .
Eventually , a strong AGN driven wind starts to clear up the ISM of dust and gas , damping the star formation and un-obscuring the line of sight toward the QSO .
In this scenario , all the QSOs hosts have final stellar masses in the range ( 4 - 6 ) \times 10 ^ { 11 } M _ { \odot } , a factor 3-30 larger than the upper limits allowed by the observations .
We discuss alternative scenarios to alleviate this apparent tension : the most likely explanation resides in the large uncertainties that still affect dynamical mass measurements in these high-z galaxies .
In addition , during the transition between the starburst-dominated and the active QSO phase , we predict that \sim 40 \% of the progenitor galaxies can be classified as Sub Millimeter Galaxies , although their number rapidly decreases with redshift .