External versus internal triggers of bar formation in cosmological
  zoom-in simulations
  The emergence of a large-scale stellar bar is one of the most striking
features in disc galaxies. By means of state-of-the-art cosmological zoom-in
simulations, we study the formation and evolution of bars in Milky Way-like
galaxies in a fully cosmological context, including the physics of gas
dissipation, star formation, and supernova feedback. Our goal is to
characterise the actual trigger of the non-axisymmetric perturbation that leads
to the strong bar observable in the simulations at z=0, discriminating between
an internal/secular versus an external/tidal origin. To this aim, we run a
suite of cosmological zoom-in simulations altering the original history of
galaxy-satellite interactions at a time when the main galaxy, though already
bar-unstable, does not feature any non-axisymmetric structure yet. We find that
the main effect of a late minor merger and of a close fly-by is to delay the
time of bar formation and those two dynamical events are not directly
responsible for the development of the bar and do not alter significantly its
global properties (e.g. its final extension). We conclude that, once the disc
has grown to a mass large enough to sustain global non-axisymmetric modes, then
bar formation is inevitable.
