Abstract

The mass assembly of galaxies leaves various imprints in their surroundings, such as shells, streams, tidal tails, tidal dwarf galaxies, extended stellar halos, etc. The frequency and properties of these structures depend on the mechanism driving the mass assembly: rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry / wet mergers. Therefore by studying the outskirts of galaxies, one can learn about their main formation mechanism. I will present our on-going work to characterize the external regions of about 400 massive nearby galaxies based on ultra-deep wide-field optical images obtained as part of the MATLAS and NGVS projects at the CFHT . We use the diffuse light to extend outside the Local Group the technique of galaxy archeology traditionally made with stellar counts. Initial results pertain to the discovery of a wealth of low surface brightness structures, including collisional debris as well as unexpected blue star-forming disks around ellipticals and red halos around spirals, and to instructive correlations between the level of tidal perturbation, and the mass, dynamical state and gas content of the host galaxies. I will finally discuss the prospect of deep optical imaging to not only study the structuration of galaxies, but also that of the interstellar medium.