An efficient positive potential-density pair expansion for modelling galaxies

TL;DR

This paper introduces a new positive potential-density pair expansion method using Miyamoto-Nagai discs, enabling efficient and accurate galaxy modeling across various density profiles, including exponential and triaxial models.

Contribution

The authors develop a novel positive potential-density expansion using MN discs with negative scale lengths, ensuring global positivity and improving galaxy modeling efficiency.

Findings

Achieves better than 10% density reconstruction accuracy

Represents exponential discs with 4 MNn discs over 0.1-10 scale lengths

Models twisted triaxial NFW profiles with 3 MNn discs per axis

Abstract

We present a novel positive potential-density pair expansion for modelling galaxies, based on the Miyamoto-Nagai (MN) disc. By using three sets of such discs, each one of them aligned along each symmetry axis, we are able to reconstruct a broad range of potentials that correspond to density profiles from exponential discs to 3D power law models with varying triaxiality (henceforth simply "twisted" models). We increase the efficiency of our expansion by allowing the scale length parameter of each disc to be negative. We show that, for suitable priors on the scale length and height parameters, these "MNn discs" have just one negative density minimum. This allows us to ensure global positivity by demanding that the total density at the global minimum is positive. We find that at better than 10\% accuracy in our density reconstruction, we can represent a radial and vertical exponential disc over $0.1-10$ scale lengths/heights with 4 MNn discs, an NFW profile over $0.1-10$ scale lengths with 4 MNn discs, and a twisted triaxial NFW profile with 3 MNn discs per symmetry axis. Our expansion is efficient, fully analytic, and well-suited to reproducing the density distribution and gravitational potential of galaxies from discs to ellipsoids.