Evolution of galaxies: interactions, mergers, and gas accretion
Studying present day properties of galaxies can give information both on their past evolution and on the matter content of the Universe. Morphological and kinematical properties can be studied statistically, as a function of environment, with recent observations. To interpret these observed properties, we have modelled the formation and past evolution of galaxies with numerical simulations. The codes that are used, already existing for a part, and developed during the thesis for the other part, model the gravitational dynamics of stars, gas clouds and dark matter, the dissipative hydrodynamic nature of interstellar gas, and star formation. Outputs from the simulations can then be compared to observations to understand which processes have led to the current state of galaxies. In a first part, we study the morphology of isolated galaxies. We show that most spiral galaxies have a bar -- an elongated central concentration of stars. This bar should have been destroyed in a few billions year by angular momentum transfer to the interstellar medium. The ubiquity of bars since 10 billion years then requires them to be reformed or strengthen by an external process. This can be explained only if galaxies accrete large amounts of gas from the intergalactic medium. The study of disk lopsidedness in spiral galaxies also argues towards this conclusion. We estimate the typical accretion rate, of w few solar masses per year for a Milky-Way like galaxy. This give strong constraints for cosmological models: the Universe must contain enough baryonic matter, tat must not be converted into stars too rapidly, so that enough gas remains to be accreted and increase the mass of spiral galaxies by a few tens of percent below the epoch of redshift z=1. Then, galaxies have grown by a gas accretion process, for a part, by collisions and mergers also play an important role in their evolution. It is well-known that equal-mass galaxy mergers from elliptical galaxies. We show that even mergers will small companions strongly affect the disk of spiral galaxies. Indeed, many mergers over a large range of mass ratios can convert spiral galaxies into lenticular or elliptical systems; the accretion of gas evoked before can reform a thin spiral disk later-on. We also demonstrate that multiple successive minor mergers with dwarf galaxies can lead to the formation of elliptical galaxies, as a major merger with a same-mass galaxy do. This new process for the formation of elliptical galaxies should lead to a revision of estimates of the dark matter content in elliptical galaxies, which may give new information on the nature of this dark matter itself. Other phenomenons occur during galaxy collisions (formation of rings, genesis of dwarf galaxies in tidal tails). They have been studied with the help of our numerical models. They help to constrain the properties and nature of dark matter, by indicating how it behaves during galaxy interactions. The comparison of numerical models to observational data indicates that a part of dark matter if condensed into the collisional and tidal debris. Even if higher resolution observations re still required to reach a definitive conclusion, this tends to indicate that a part of dark matter has a collisional dynamics -- probably the baryonic component of dark matter, which favour the hypothesis of dark molecular gas. The statistical comparison of observations and high-resolution numerical simulations has thus given constraints on the physical process that drive galaxy evolution, and the visible and dark matter content of the Universe. In the future, increase capability of observation and numerical simulation, dedicated in particular to distant galaxies, will enable an even better understanding of galaxy evolution, large-scale star formation, and the history of the baryonic content of the Universe. Cosmological scenarios for the evolution of the whole Universe can be tested and/or refined with the help of such studies.