Joint cosmological formation of QSOs and bulge-dominated galaxies

Older and more recent pieces of observational evidence suggest a strong con nection between QSOs and galaxies; in particular, the recently discovered c orrelation between black hole and galactic bulge masses suggests that QSO a ctivity is directly connected to the formation of galactic bulges. The cosm ological problem of QSO formation is analysed in the framework of an analyt ical model for galaxy formation; for the first time a joint comparison with galaxy and QSO observables is performed. In this model it is assumed that the same physical variable that determines galaxy morphology is able to mod ulate the mass of the black hole responsible for QSO activity. Both halo sp in and the occurrence of a major merger are considered as candidates for th is role. The predictions of the model are compared with available data for the type-dependent galaxy mass functions, the star formation history of ell iptical galaxies, the QSO luminosity function and its evolution (including the obscured objects contributing to the hard-X-ray background), the mass f unction of dormant black holes and the distribution of black hole-to-bulge mass ratios. A good agreement with observations is obtained if the halo spi n modulates the efficiency of black hole formation, and if the galactic hal oes at z = 0 have shone in an inverted order with respect to the hierarchic al one (i.e., stars and black holes in bigger galactic haloes have formed b efore those in smaller ones). This inversion of hierarchical order for gala xy formation, which reconciles galaxy formation with QSO evolution, is cons istent with many pieces of observational evidence.