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The spherical Jeans equation (SJE) is widely used in dynamical modelling ofthe Milky Way (MW) halo potential. We use haloes and galaxies from thecosmological Millennium-II simulation and hydrodynamical APOSTLE simulations toinvestigate the performance of the SJE in recovering the underlying massprofiles of MW mass haloes. The best-fitting halo mass and concentrationparameters scatter by 25% and 40% around their input values, respectively, whendark matter particles are used as tracers. This scatter becomes as large as afactor of 3 when using star particles instead. This is significantly largerthan the estimated statistical uncertainty associated with the use of the SJE.The existence of correlated phase-space structures that violate the steadystate assumption of the SJE as well as non-spherical geometries are theprincipal sources of the scatter. Binary haloes show larger scatter becausethey 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 ofindependent phase-space structures sets an intrinsic limiting precision ondynamical inferences based on the steady state assumption. Modelling with aradius-independent velocity anisotropy, or using tracers within a limited outerradius, result in significantly larger scatter, but the ensemble-averagedmeasurement over the whole halo sample is approximately unbiased.

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