Monica Valluri, Victor P. Debattista, Gregory S. Stinson, Jeremy Bailin, Thomas R. Quinn, H. M. P. Couchman, James Wadsley
We analyze the orbits of stars and dark matter particles in the halo of a disk galaxy formed in a cosmological hydrodynamical simulation. Although the halo is oblate within the inner 20 kpc and only mildly triaxial beyond this radius, only 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 this nearly oblate halo is comprised primarily of axisymmetric short-axis tube orbits. Orbits of all families 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 identified in plots of metallicity versus formation time. Dynamically young tidal debris is found primarily on tube orbits, suggesting that accretions probably occur mainly on such orbits. However, stars associated with older satellites have 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. This implies that the dynamically older debris has experienced chaotic scattering during the evolution, and individual progenitors can no longer be uniquely identified using phase space coordinates alone. Thus "galactic archeology" in the fully cosmological context requires the ability to correlate information on stellar ages and chemical abundances with the orbital properties of halo stars in the underlying self-consistent potential.
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http://arxiv.org/abs/1301.4517
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