The Atlas-3D project - IX. The merger origin of a fast and a slow
  rotating Early-Type Galaxy revealed with deep optical imaging: first results
  The mass assembly of galaxies leaves imprints in their outskirts, such as
shells and tidal tails. The frequency and properties of such fine structures
depend on the main acting mechanisms - secular evolution, minor or major
mergers - and on the age of the last substantial accretion event. We use this
to constrain the mass assembly history of two apparently relaxed nearby
Early-Type Galaxies (ETGs) selected from the Atlas-3D sample, NGC 680 and NGC
5557. Our ultra deep optical images obtained with MegaCam on the
Canada-France-Hawaii Telescope reach 29 mag/arcsec^2 in the g-band. They reveal
very low-surface brightness (LSB) filamentary structures around these
ellipticals. Among them, a gigantic 160 kpc long tail East of NGC 5557 hosts
gas-rich star-forming objects. NGC 680 exhibits two major diffuse plumes
apparently connected to extended HI tails, as well as a series of arcs and
shells. Comparing the outer stellar and gaseous morphology of the two
ellipticals with that predicted from models of colliding galaxies, we argue
that the LSB features are tidal debris, the star-forming objects near NGC 5557,
long lived Tidal Dwarf Galaxies and that each of the two ETGs was assembled
during a relatively recent, major wet merger, which likely occurred at a
redshift below z = 0.5. The inner kinematics of NGC 680 is typical for fast
rotators which make the bulk of nearby ETGs in the Atlas-3D sample. NGC 5557
belongs to the poorly populated class of massive, round, slow rotators that are
predicted by semi-analytic models and cosmological simulations to be the
end-product of a complex mass accretion history, involving ancient major
mergers and more recent minor mergers. Our observations suggest that under
specific circumstances a single binary merger may dominate the formation
history of such objects and thus that at least some massive ETGs may form at
relatively low redshift (abridged).
