Using deep images and simulations to trace collisional debris around
  massive galaxies
  Deep imaging programs, such as MATLAS which has just been completed at the
CFHT, allows us to study with their diffuse light the outer stellar populations
around large number of galaxies. We have carried out a systematic census of
their fine structures, i.e. the collisional debris from past mergers. We have
identified among them stellar streams from minor mergers, tidal tails from
gas-rich major mergers, plumes from gas-poor major mergers, and shells from
intermediate mass mergers. Having estimated the visibility and life time of
each of these structures with numerical simulations, we can reconstruct the
past mass assembly of the host galaxy. Preliminary statistical results based on
a sample of 360 massive nearby galaxies are presented.
