The mass assembly of galaxies, especially the most massive ones, leaves various imprints in their surroundings, such as shells, streams, tidal tails, tidal dwarf galaxies, etc... The frequency and properties of these fine structures depend on the mechanism driving the mass assembly. Depending on whether massive galaxies, were formed through a monolithic collapse, rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry / wet mergers, the ubiquity of fine structures will go from none to significant. 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 parts of massive nearby early type galaxies which relies on :
- stunning extremely deep, wide-field, optical images obtained at CFHT of field and cluster ETGs obtained as part of the ATLAS-3D, MATLAS and NGVS projects, which reveal a wealth of unknown faint structures at levels as faint as 29 mag/arcsec2.
- state of the art numerical simulations of mergers, either idealized or made in cosmological context, which are used to interpret the observational data and constrain the mass assembly of galaxies.