Galaxy Formation:the role of gas accretion

In the hierarchical model, small galaxies form first and merge together into more massive objects. In addition, numerical simulations recently emphasized the role of smooth gas accretion. Using state-of-the-art cosmological hydrodynamical multi-zoom simulations, we studied the formation and evolution of galaxies in a dense environment. We detected the structures in the simulations, both for dark matter haloes and baryonic galaxies. For the latter, we explored the parameter space of the structure finder to be able to detect galaxies. We then built the merger trees of the haloes and the galaxies.We focused on the properties of the haloes in this dense environment, studying the effect of the environment on the mass function as well as its evolution in time. We found a hierarchical trend, with low mass structures forming first, and merging into more massive ones. We studied then the baryon contents of the haloes, and the evolution of the baryons in stars, condensed, diffuse and hot gas. Finally, we studied the relation between the mass and the velocity dispersion of galaxies and haloes.In a second part, we studied the mass assembly of galaxies, distinguishing between the mass smoothly accreted and the mass assembled by mergers. We followed the mass assembly of all our galaxies, and found that it is dominated by smooth accretion. We also found a downsizing trend, with massive galaxies having older stars than less massive ones. This downsizing is however not incompatible with the hierarchical scenario