Joint Formation of QSOs and Spheroids: QSOs as clocks of star formation in Spheroids

Abstract

Direct and indirect observational evidence leads to the conclusion that high redshift QSOs did shine in the core of early type galaxies during their main episode of star formation. In a previous paper we showed that the evolution of the luminosity function of QSOs can be accounted for within the framework of the analytical hierarchical model for dark matter haloes in the line of the extended Press-Schechter formalism. Observational evidences suggest that the hierarchical order, which holds for dark haloes, is inverted in the processes involving baryons. In this paper we present our model predictions for submillimeter galaxy counts and their follow-ups and we point out that the comparison with available data, in particular with the source counts at 850 and 450 $\\mu$m, confirms the tight link between star formation and the BH shining phase in elliptical galaxies. In particular, data point to a model in which at a fiducial time t_{sh} the QSOs reach luminosities high enough to trigger galactic winds, which remove the gas from the galaxies, stopping not only accretion on the central BHs but also star formation. The winds occur earlier in larger galaxies, which host more massive BHs. This sequence reproduces the main properties of elliptical galaxies. The winds powered by the QSOs set up a correlation between the mass of the central BH and that of the bulge of the host galaxy, as observed. In this scenario elliptical galaxies first appear as ultraluminous far-IR objects, extremely faint and quite red in the optical bands; they are seen at high redshifts at 850 $\\mu$m thanks to the very favourable K-correction. Later, the QSOs shine for a short time. As for the post-starburst phase, bright Lyman-break galaxies are the most likely optical counterparts at $z\\gtrsim 3$. A long passive evolution eventually follows.