The Sagittarius Stream and Halo Triaxiality

TL;DR

This paper models the Milky Way's mass distribution, fitting observational data of the Sagittarius stream with a triaxial dark matter halo, revealing a highly aspherical halo that challenges standard cold dark matter predictions.

Contribution

It provides a detailed Bayesian analysis of the Milky Way's mass model, including a triaxial halo aligned with the Sagittarius stream, which is a novel constraint on galaxy halo shapes.

Findings

Best-fit halo is triaxial with specific axis ratios

Halo shape aligns with the Sun-Galactic centre line

Results challenge standard cold dark matter models

Abstract

We present a mass model for the Milky Way, which is fit to observations of the Sagittarius stream together with constraints derived from a wide range of photometric and kinematic data. The model comprises a S\'{e}rsic bulge, an exponential disk, and an Einasto halo. Our Bayesian analysis is accomplished using an affine-invariant Markov chain Monte Carlo algorithm. We find that the best-fit dark matter halo is triaxial with axis ratios of $3.3\pm 0.7$ and $2.7\pm 0.4$ and with the short axis approximately aligned with the Sun-Galactic centre line. Our results are consistent with those presented in Law and Majewski (2010). Such a strongly aspherical halo is disfavoured by the standard cold dark matter scenario for structure formation.