Local variations of the Stellar Velocity Ellipsoid-I: the disc of
  galaxies in the Auriga simulations
  The connection between the Stellar Velocity Ellipsoid (SVE) and the dynamical
evolution of galaxies has been a matter of debate in the last years and there
is no clear consensus whether different heating agents (e.g. spiral arms, giant
molecular clouds, bars and mergers) leave clear detectable signatures in the
present day kinematics. Most of these results are based on a single and global
SVE and have not taken into account that these agents do not necessarily
equally affect all regions of the stellar disc.We study the 2D spatial
distribution of the SVE across the stellar discs of Auriga galaxies, a set of
high resolution magneto-hydrodynamical cosmological zoom-in simulations, to
unveil the connection between local and global kinematic properties in the disc
region. We find very similar, global, $\sigma_{z}/\sigma_{r}$= 0.80$\pm$ 0.08
values for galaxies of different Hubble types. This shows that the global
properties of the SVE at z=0 are not a good indicator of the heating and
cooling events experienced by galaxies. We also find that similar
$\sigma_{z}/\sigma_{r}$radial profiles are obtained through different
combinations of $\sigma_{z}$ and $\sigma_{r}$ trends: at a local level, the
vertical and radial components can evolve differently, leading to similar
$\sigma_{z}/\sigma_{r}$ profiles at z=0. By contrast, the 2D spatial
distribution of the SVE varies a lot more from galaxy to galaxy. Present day
features in the SVE spatial distribution may be associated with specific
interactions such as fly-by encounters or the accretion of low mass satellites
even in the cases when the global SVE is not affected. The stellar populations
decomposition reveals that young stellar populations present colder and less
isotropic SVEs and more complex 2D distributions than their older and hotter
counterparts.
