This lecture reviews the physical processes of Interstellar Medium (ISM) dynamics and Star Formation (SF) in galaxy interactions and mergers. Galaxy collisions and mergers are one of the fundamental processes of galaxy assembly in -CDM cosmology. Mergers are often associated to "starbursts" where the rate of star formation can increase by a larger factor, and the gas consumption timescale drops. Here we aim at explaining the observed properties of these merger-driven starbursts with a theoretical point of view and with the help of hydrodynamic simulations, and compare to isolated disk galaxies that form stars quiescently.
The fundamental processes of star formation in any galaxy are firs reviewed in a brief and simple way - a thorough review of these is provided by Elmegreen (this volume; arxiv:1101.3108 to 1101.3113). The role of ISM turbulence is highlighted. We then study the dynamics of galaxy interactions and mergers, and the general properties of SF in mergers. The standard theory for merger-induced starbursts, based on tidally-driven gas inflows triggering a nuclear starbursts, is reviewed and its limitations are pointed out - real starbursts tend to be stronger, and more extended spatially. We show with modern high-resolution simulations that another triggering process is the increase of ISM turbulence and fragmentation, resulting from the interactions and inducing an excess of very dense, rapidly star-forming molecular gas. Merger-induced starbursts then have a nuclear component, but also an extended component in massive and dense star clusters, as generally observed. The gas compression is thus both global (over the whole galaxy) from the tidal field, and local (on scales of 0.1-1 kpc) from local shocks in the turbulence ISM. This explains recent observations, such as a dual law of star formation between quiescent disks and starbursting mergers, and an excess of very dense molecular gas in Ultra Luminous Infrared galaxies.
Structure formation and (globular) star cluster formation/evolution, also included in this lecture, are briefly summarized at the end of the present text, and the reader is referred to Bournaud (2010a) for more details on these aspects.