HALO ORBITS IN COSMOLOGICAL DISK GALAXIES: TRACERS OF FORMATION HISTORY
- Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States)
- Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE (United Kingdom)
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
- Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487-0324 (United States)
- Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195-1580 (United States)
- Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1 (Canada)
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 {approx}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 {approx}> 0.6. We find that randomly selected samples of halo stars show no substructure in 'integrals of motion' space. However, individual accretion events can clearly be 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.
- OSTI ID:
- 22167381
- Journal Information:
- Astrophysical Journal, Vol. 767, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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