Wednesday, June 26, 2013

1306.6016 (A. Bowden et al.)

Triaxial Cosmological Haloes and the Disc of Satellites    [PDF]

A. Bowden, N. W. Evans, V. Belokurov
We construct simple triaxial generalisations of Navarro-Frenk-White haloes. The models have elementary gravitational potentials, together with a density that is cusped like 1/r at small radii and falls off like 1/r^3 at large radii. The ellipticity varies with radius in a manner that can be tailored to the user's specification. The closed periodic orbits in the planes perpendicular to the short and long axes of the model are well-described by epicyclic theory, and can be used as building blocks for long-lived discs. As an application, we carry out the simulations of thin discs of satellites in triaxial dark halo potentials. This is motivated by the recent claims of an extended, thin disc of satellites around the M31 galaxy with a vertical rms scatter of ~12 kpc and a radial extent of ~ 300 kpc (Ibata et al. 2013). We show that a thin satellite disc can persist over cosmological times if and only if it lies in the planes perpendicular to the long or short axis of a triaxial halo, or in the equatorial or polar planes of a spheroidal halo. In any other orientation, then the disc thickness doubles on ~5 Gyr timescales and so must have been born with an implausibly small vertical scaleheight. A natural consequence of a thin disc of satellites which contains the Milky Way galaxy is that the latter's tidal forces are very effective in distorting the orbits. Discs of satellites that are initially nearly circular become lop-sided and distended towards the Milky Way, even after a few dynamical timescales. This provides a natural explanation of the long-standing puzzle that the observed distribution of M31 satellites lies preponderantly on the near side with respect to the Milky Way,
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