The Sagittarius stream and halo triaxiality

We present a mass model for the Milky Way, which is fitted to observations of the Sagittarius stream together with constraints derived from a wide range of photometric and kinematic data. The model comprises a Sérsic bulge, an exponential disc and an Einasto halo. Our Bayesian analysis is accomplished using an affine-invariant Markov chain Monte Carlo algorithm. We find that the best-fitting dark matter halo is triaxial with axis ratios of 3.3 ± 0.7 and 2.7 ± 0.4 and with the short axis approximately aligned with the Sun–Galactic Centre line. Our results are consistent with those presented in Law & Majewski. Such a strongly aspherical halo is disfavoured by the standard cold dark matter scenario for structure formation.