Exact potential-density pairs for flattened dark haloes

TL;DR

This paper develops and refines a method to convert spherical dark matter halo models into flattened, axisymmetric ones, preserving key density profile features, but with limitations at large radii and for strong flattening.

Contribution

It formalizes an extension of Miyamoto & Nagai's method for arbitrary potential-density pairs and introduces modifications to improve large-radius behavior.

Findings

The method preserves inner density slopes during flattening.

Original method shows disc-like features at large radii, limiting its applicability.

Modified approach yields more realistic density profiles at large radii.

Abstract

Cosmological simulations suggest that dark matter haloes are not spherical, but typically moderately to strongly triaxial systems. We investigate methods to convert spherical potential-density pairs into axisymmetric ones, in which the basic characteristics of the density profile (such as the slope at small and large radii) are retained. We achieve this goal by replacing the spherical radius r by an oblate radius m in the expression of the gravitational potential. We extend and formalize the approach pioneered by Miyamoto & Nagai (1975) to be applicable to arbitrary potential-density pairs. Unfortunately, an asymptotic study demonstrates that, at large radii, such models always show a R^(-3) disc superposed on a smooth roughly spherical density distribution. As a result, this recipe cannot be used to construct simple flattened potential-density pairs for dynamical systems such as dark matter haloes. Therefore we apply a modification of our original recipe that cures the problem of the discy behaviour. An asymptotic analysis now shows that the density distribution has the desired asymptotic behaviour at large radii (if the density falls less rapidly than r^(-4)). We also show that the flattening procedure does not alter the shape of the density distribution at small radii: while the inner density contours are flattened, the slope of the density profile is unaltered. We apply this recipe to construct a set of flattened dark matter haloes based on the realistic spherical halo models by Dehnen & McLaughlin (2005). This example illustrates that the method works fine for modest flattening values, whereas stronger flattening values lead to peanut-shaped density distributions.