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
semianalytic models. As for the dark matter (DM), we follow the detailed
evolution of halos within the canonical LCDM cosmology using standard
Montecarlo 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
onto the halo outskirts. We propose that the two modes of halo growth drive two
distinct modes for the evolution of baryonic matter, favoring the development
of the spheroidal and disc components of galaxies, respectively. We test this
idea using the semianalytic 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 to the spectrophotometric code GRASIL, and compare our results on
several properties of the local galaxy population with observations, Finding an
encouraging agreement.
