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Milking the spherical cow - on aspherical dynamics in spherical coordinates

Pontzen, Andrew ; Read, Justin I. ; Teyssier, Romain ; Governato, Fabio ; Gualandris, Alessia ; Roth, Nina ; Devriendt, Julien

In: Monthly Notices of the Royal Astronomical Society, 2015, vol. 451, no. 2, p. 1366-1379

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    Titre
    • Milking the spherical cow - on aspherical dynamics in spherical coordinates
    Auteur
    • Pontzen, Andrew. Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
    • Read, Justin I.. Department of Physics, University of Surrey, Guildford, GU2 7XH Surrey, UK
    • Teyssier, Romain. Institute for Theoretical Physics, University of Zurich, CH-8057 Zürich, Switzerland
    • Governato, Fabio. Astronomy Department, University of Washington, Seattle, WA 98195, USA
    • Gualandris, Alessia. Department of Physics, University of Surrey, Guildford, GU2 7XH Surrey, UK
    • Roth, Nina. Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
    • Devriendt, Julien. Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • Monthly Notices of the Royal Astronomical Society, 2015, vol. 451, no. 2, p. 1366-1379. Oxford University Press
    Autre version électronique
    Identifiant OAI-PMH
    • oai:doc.rero.ch:294673
    Summary
    Galaxies and the dark matter haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealized calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation? We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are ‘maximally stable', i.e. that do not evolve at first order when external potentials (which arise from baryons, large-scale tidal fields or infalling substructure) are applied. We show that a spherically symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealized spherically symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter haloes (formed in collisionless cosmological simulations) nearly attain our maximally stable limit, but their evolution freezes out before reaching it