What to expect from dynamical modelling of galactic haloes II: the spherical Jeans equation

TL;DR

This study evaluates the spherical Jeans equation's effectiveness in recovering Milky Way halo mass profiles using cosmological simulations, revealing significant scatter due to phase-space structures and halo shape complexities.

Contribution

It quantifies the limitations of the spherical Jeans equation in realistic halo conditions, highlighting the impact of phase-space correlations and non-sphericity on mass inference accuracy.

Findings

Mass and concentration estimates scatter by 25-40% with dark matter tracers.

Using star particles increases scatter up to a factor of 3.

Intrinsic phase-space structures limit the precision of dynamical inferences.

Abstract

The spherical Jeans equation (SJE) is widely used in dynamical modelling of the Milky Way (MW) halo potential. We use haloes and galaxies from the cosmological Millennium-II simulation and hydrodynamical APOSTLE simulations to investigate the performance of the SJE in recovering the underlying mass profiles of MW mass haloes. The best-fitting halo mass and concentration parameters scatter by 25% and 40% around their input values, respectively, when dark matter particles are used as tracers. This scatter becomes as large as a factor of 3 when using star particles instead. This is significantly larger than the estimated statistical uncertainty associated with the use of the SJE. The existence of correlated phase-space structures that violate the steady state assumption of the SJE as well as non-spherical geometries are the principal sources of the scatter. Binary haloes show larger scatter because they are more aspherical in shape and have a more perturbed dynamical state. Our results confirm the previous study of Wang et al. (2017) that the number of independent phase-space structures sets an intrinsic limiting precision on dynamical inferences based on the steady state assumption. Modelling with a radius-independent velocity anisotropy, or using tracers within a limited outer radius, result in significantly larger scatter, but the ensemble-averaged measurement over the whole halo sample is approximately unbiased.