  The mass assembly of galaxies leaves various imprints on their surroundings,
such as shells, streams and tidal tails. The frequency and properties of these
fine structures depend on the mechanism driving the mass assembly: e.g. a
monolithic collapse, 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 present here our on-going work to characterize the outskirts of Early-Type
Galaxies (ETGs), which are powerful probes at low redshift of the hierarchical
mass assembly of galaxies. This work relies on ultra-deep optical images
obtained at CFHT with the wide-field of view MegaCam camera of field and
cluster ETGs obtained as part of the Atlas-3D and NGVS projects. State of the
art numerical simulations are used to interpret the data. The images reveal a
wealth of unknown faint structures at levels as faint as 29 mag arcsec-2 in the
g-band. Initial results for two galaxies are presented here.
