It is an undeniable fact that tidal forces and the formation of tidal tails are overall a second order process in galaxy evolution. The fraction of stars expelled in the intergalactic medium is low, at most a few percent in major mergers. The fraction of gas is more important, but the bulk of the gaseous reservoir is funneled into the central regions. Collisional debris may host star-forming regions, but their contribution to the total star formation rate is minimum. Clearly, most of the activity occurs in the more central and nuclear regions where starbursts and/or AGN fueling is triggered. However, one of the aims of the present Review is to emphasize the idea that tidal debris can provide insightful information about the properties of galaxies, the same way as garbage in trash cans tells us much about the way of life of their owners.
The presence of tidal features is an unambiguous proof that a major/minor merger occurred in the recent past, and that at least one of the colliding galaxies had a stellar and/or gaseous disk. The converse is not true though, as not all collisions produce prominent tidal features. Determining when the merger took place is less strait-forward. However, numerical simulations done in cosmological context will soon be able to constrain the survival time of collisional debris and thus give predictions on their age. Comparisons between observations and simulations should then allow us to reconstruct the mass assembly of galaxies. Current generation of wide-field-of-view cameras and the on-going extremely deep surveys of the nearby Universe detect numerous new tidal features of very low surface brightness, offering interesting prospects to galactic archeology. At high redshift, the census of tidal perturbations is much more complex, not only because of dimming and band shifting issues, but also because distant galaxies are much more gas-rich and therefore are intrinsically irregular. This makes the separation between secular and external effects rather ambiguous.
Multi-wavelength surveys have revealed the presence in collisional debris of all the constituents of regular galaxies though with different proportions: young and old stars, atomic gas, molecular gas, even possibly dark gas, heavy elements and dust. Star formation seems to proceed there in a similar way as in isolated spiral disks, despite the very different environment at large scale.
Tidal tails may in principle even be used to probe some fundamental aspects of physics, including, of course, the properties of tidal forces but also the laws of gravitation, as shown by recent experiments with modified gravity. The fact that tidal forces can be compressive and for instance contribute to the stability of star clusters whereas they are usually associated with destruction processes has only recently been understood. The shape of the tidal tensor explains why massive tidal dwarf galaxies may only be formed within an extended dark matter halo. A theoretical study on the nature and the role of tidal forces in galaxies remains largely to be done and might provide further surprises.
First of all, we express our gratitude to the two main organizers of the school, Jean Souchay and Stéphane Mathis. We not only enjoyed the marvelous premises - Cargese in Corsica -, but also the very stimulating discussions that took place between experts in terrestrial, planetary, stellar and galactic tides. We are very grateful to Frédéric Bournaud for daily discussions on galaxy collisions and numerical simulations. We finally wish to thank all our collaborators and colleagues for their crucial contributions to the various works presented here.