Self-consistent triaxial models

TL;DR

This paper introduces the first self-consistent triaxial stellar models with analytic distribution functions, capable of modeling elliptical galaxies and dark haloes, and exhibiting realistic features like isophote twisting.

Contribution

The authors develop analytic distribution functions for self-consistent triaxial models, including profiles with cores or cusps, and analyze their orbital structure and stability.

Findings

Models reproduce observed isophote twists with a gradient of ~10°/effective radius.

Triaxial models are linked to radial orbit instability.

Outer anisotropy influences triaxiality and stability.

Abstract

We present self-consistent triaxial stellar systems that have analytic distribution functions (DFs) expressed in terms of the actions. These provide triaxial density profiles with cores or cusps at the centre. They are the first self-consistent triaxial models with analytic DFs suitable for modelling giant ellipticals and dark haloes. Specifically, we study triaxial models that reproduce the Hernquist profile from Williams & Evans (2015), as well as flattened isochrones of the form proposed by Binney (2014). We explore the kinematics and orbital structure of these models in some detail. The models typically become more radially anisotropic on moving outwards, have velocity ellipsoids aligned in Cartesian coordinates in the centre and aligned in spherical polar coordinates in the outer parts. In projection, the ellipticity of the isophotes and the position angle of the major axis of our models generally changes with radius. So, a natural application is to elliptical galaxies that exhibit isophote twisting. As triaxial St\"ackel models do not show isophote twists, our DFs are the first to generate mass density distributions that do exhibit this phenomenon, typically with a gradient of $\approx 10^\circ$/effective radius, which is comparable to the data. Triaxiality is a natural consequence of models that are susceptible to the radial orbit instability. We show how a family of spherical models with anisotropy profiles that transition from isotropic at the centre to radially anisotropic becomes unstable when the outer anisotropy is made sufficiently radial. Models with a larger outer anisotropy can be constructed but are found to be triaxial. We argue that the onset of the radial orbit instability can be identified with the transition point when adiabatic relaxation yields strongly triaxial rather than weakly spherical endpoints.