Two-phase galaxy formation

We propose and test a scenario for the assembly and evolution of luminous matter in galaxies which substantially differs from that adopted by other semi-analytic models. As for the dark matter, we follow the detailed evolution of haloes within the canonical Λ cold dark matter cosmology using standard Monte Carlo methods. However, when overlaying prescriptions for baryon evolution, we take into account an effect pointed out in the past few years by a number of studies mostly based on intensive N-body simulations, namely that typical halo growth occurs in two phases: an early, fast collapse phase featuring several major merger events, followed by a late, quiescent accretion on to the halo outskirts. We propose that the two modes of halo growth drive two distinct modes for the evolution of baryonic matter, favouring the development of the spheroidal and disc components of galaxies, respectively. We test this idea using the semi-analytic technique. Our galaxy formation model envisages an early coevolution of spheroids and the central supermassive black holes, already tested in our previous works, followed by a relatively quiescent growth of discs around the preformed spheroids. In this exploratory study, we couple our model with the spectrophotometric code GRASIL, and compare our results on several properties of the local galaxy population with observations, finding an encouraging agreement.