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