Triaxial Cosmological Haloes and the Disc of Satellites

TL;DR

This paper develops triaxial dark matter halo models with simple gravitational potentials and explores the stability of thin satellite discs within these halos, providing insights into the observed satellite distribution around M31.

Contribution

It introduces flexible triaxial halo models with analytical potentials and studies the long-term stability of satellite discs in various halo orientations.

Findings

Thin satellite discs can persist over cosmological times only in specific halo orientations.

Discs in non-perpendicular planes to halo axes tend to thicken within ~5 Gyr.

The models help explain the observed thin disc of satellites around M31.

Abstract

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.