Halo orbits in cosmological disk galaxies: tracers of formation history
  We analyze the orbits of stars and dark matter particles in the halo of a
disk galaxy formed in a cosmological hydrodynamical simulation. The halo is
oblate within the inner ~20 kpc and triaxial beyond this radius. About 43% of
orbits are short axis tubes - the rest belong to orbit families that
characterize triaxial potentials (boxes, long-axis tubes and chaotic orbits),
but their shapes are close to axisymmetric. We find no evidence that the
self-consistent distribution function of the nearly oblate inner halo is
comprised primarily of axisymmetric short-axis tube orbits. Orbits of all
families, and both types of particles are highly eccentric with mean
eccentricity >0.6. We find that randomly selected samples of halo stars show no
substructure in "integrals of motion" space. However individual accretion
events can be clearly identified in plots of metallicity versus formation time.
Dynamically young tidal debris is found primarily on a single type of orbit.
However, stars associated with older satellites become chaotically mixed during
the formation process (possibly due to scattering by the central bulge and
disk, and baryonic processes), and appear on all four types of orbits. We find
that the tidal debris in cosmological hydrodynamical simulations experiences
significantly more chaotic evolution than in collisionless simulations, making
it much harder to identify individual progenitors using phase space coordinates
alone. However by combining information on stellar ages and chemical abundances
with the orbital properties of halo stars in the underlying self-consistent
potential, the identification of progenitors is likely to be possible.
