Dark matter halo mergers and quasars

The formation and evolution of galaxies and the supermassive black holes they harbor at their nuclei depends strongly on the merger history of their surrounding dark matter haloes. First we developed a semi-analytic algorithm that describes the merger history tree of a halo. The following tests were performed: the conditional mass function, the time and mass distributions at formation, and the time distribution of the last major merger. We provide a model for the creation rate of dark matter haloes, informed by both coagulation theory and the modified excursion set approach with moving barriers. A comparison with N-body simulations shows that our square-root barrier merger rate is significantly better than the standard extended Press-Schechter rate used in the literature. The last chapter is dedicated to a simple model of quasar activation by major mergers of dark matter haloes. The model consists of two main ingredients: the halo merger rate describing triggering, and a quasar light curve, which tracks the evolution of individual quasars. In this model, the mass of the seed black hole at triggering is assumed to be a fixed fraction of its mass at the peak luminosity. The light curve has two components: an exponential ascending phase and a power-law descending phase that depends on mass of the host. We postulate a self-regulation condition between the peak luminosity of the active galactic nucleus (AGN) and the mass of the host halo at triggering. This type of model for quasar evolution is at the heart of the latest semianalytic models (SAMs) of galaxy formation and it is therefore definitely worth studying in some detail. By carefully revisiting some of the main issues linked to this approach, we were able to derive several interesting and physically meaningful constraints regarding black hole evolution. We expect simple, yet observationally-constrained models like ours to play a central role in future models of galaxy formation.