Active galactic nuclei in cosmological simulations - I. Formation of black holes and spheroids through mergers

This is the first paper of a series on the methods and results of the Active Galactic Nuclei In Cosmological Simulations (AGNICS) project, which incorporates the physics of active galactic nuclei (AGNs) into Galaxies In Cosmological Simulations (GalICS), a galaxy formation model that combines large cosmological N-body simulations of dark matter hierarchical clustering and a semi-analytic approach to the physics of the baryons. The project explores the quasar-galaxy link in a cosmological perspective, in response to growing observational evidence for a close relation between supermassive black holes (SMBHs) and spheroids. The key problems are the quasar fuelling mechanism, the origin of the black hole (BH)-to-bulge mass relation, the causal and chronological link between BH growth and galaxy formation, the properties of quasar hosts and the role of AGN feedback in galaxy formation. This first paper has two goals. The first is to describe the general structure and assumptions that provide the framework for the AGNICS series. The second is to apply AGNICS to studying the joint formation of SMBHs and spheroids in galaxy mergers. We investigate under what conditions this scenario can reproduce the local distribution of SMBHs in nearby galaxies and the evolution of the quasar population. AGNICS contains two star formation modes: a quiescent mode in discs and a starburst mode in proto-spheroids, the latter triggered by mergers and disc instabilities. Here we assume that BH growth is linked to the starburst mode. The simplest version of this scenario, in which the BH accretion rate. (M). and the star formation rate in the starburst component. (M)(*burst) are simply related by a constant of proportionality, does not to reproduce the cosmic evolution of the quasar population. A model in which. (M) alpha P-burst(zeta).M-*burst where Pburst is the density of the gas in the starburst and zeta similar or equal to 0.5, can explain the evolution of the quasar luminosity function in B band and X-rays (taking into account the presence of obscured AGNs inferred from X-ray studies). The scatter and the tilt that this model introduces in the BH-to-bulge mass relation are within the observational constraints. The model predicts that the quasar contribution increases with the total bolometric luminosity and that, for a given bulge mass, the most massive BHs are in the bulges with the oldest stars.