Triaxial cosmological haloes and the disc of satellites

Abstract

We construct simple triaxial generalizations 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/r3 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 byepicyclic theory, and can be used as building blocks for long-lived discs. As an application, we carry out the simulations of thin discs of satellitesin triaxial dark halo potentials. This is motivated by the recent claims ofan extended, thin disc of satellites around the M31 galaxy with a vertical rms scatter of ~12 kpc and a radial extent of ~300 kpc. We show that a thinsatellite 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, orin the equatorial or polar planes of a spheroidal halo. In any other orientation, then the disc thickness doubles on~5Gyr time-scales and so must havebeen born with an implausibly small vertical scaleheight. © 2013 The Authors.