The oblateness of the Milky Way dark matter halo from the stellar streams of NGC 3201, M68, and Palomar 5

Carles G. Palau, Jordi Miralda-Escudé

Published 2022-12-07Version 1

We explore constraints on the oblateness of the Milky Way dark matter halo inferred from fits to three stellar streams generated by the globular clusters NGC 3201, M68 (NGC 4590), and Palomar 5. Observational constraints on the gravitational potential from the stellar distribution, rotation curve, vertical velocity dispersion, and distant Milky Way satellites are included. We model the dark halo density profile as an axisymmetric two power-law model with axis ratio $q_\rho^{\rm h}$ and four additional free parameters for the radial profile. The stream of NGC 3201 alone barely constrains the halo axis ratio. The Palomar 5 stream constrains the halo to be close to spherical with $q_\rho^{\rm h}=1.01\pm0.09$. The stream of M68 is also consistent with a spherical halo but favours a prolate configuration of $q_\rho^{\rm h}=1.14^{+0.21}_{-0.14}$. The three streams together are well fitted with a halo axis ratio $q_\rho^{\rm h}=1.06\pm0.06$ and core radius $\sim$ 20 kpc. Our estimate of the halo oblateness agrees with previous studies using other observational data and is in tension with cosmological simulations predicting that most spiral galaxies have oblate dark matter halos with the short axis perpendicular to the disc. We discuss the impact of the tidal perturbation of the Magellanic Clouds, which we assume too small to change the conclusion on the axis ratio at the radii where we probe it. We note that dynamical equilibrium of a spherical halo in the presence of the disc potential implies an anisotropic dark matter velocity dispersion tensor with larger dispersion in the vertical direction than the horizontal ones, which should relate back to the assembly history of the Milky Way.