Vertical disc heating in Milky Way-sized galaxies in a cosmological
  context
  Vertically extended, high velocity dispersion stellar distributions appear to
be a ubiquitous feature of disc galaxies, and both internal and external
mechanisms have been proposed to be the major driver of their formation.
However, it is unclear to what extent each mechanism can generate such a
distribution, which is likely to depend on the assembly history of the galaxy.
To this end, we perform 16 high resolution cosmological-zoom simulations of
Milky Way-sized galaxies using the state-of-the-art cosmological
magneto-hydrodynamical code \textlcsc{AREPO}, and analyse the evolution of the
vertical kinematics of the stellar disc in connection with various heating
mechanisms. We find that the bar is the dominant heating mechanism in most
cases, whereas spiral arms, radial migration, and adiabatic heating from
mid-plane density growth are all sub-dominant. The strongest source, though
less prevalent than bars, originates from external perturbations from
satellites/sub-halos of masses log$_{10} (M/\rm M_{\odot}) \gtrsim 10$.
However, in many simulations the orbits of newborn star particles become cooler
with time, such that they dominate the shape of the age-velocity dispersion
relation and overall vertical disc structure unless a strong external
perturbation takes place.
