First Gaia dynamical model of the Milky Way disc with six phase space
  coordinates: a test for galaxy dynamics
  We construct the first comprehensive dynamical model for the high-quality
subset of stellar kinematics of the Milky Way disc, with full 6D phase-space
coordinates, provided by the Gaia Data Release 2. We adopt an axisymmetric
approximation and use an updated Jeans Anisotropic Modelling (JAM) method,
which allows for a generic shape and radial orientation of the velocity
ellipsoid, as indicated by the Gaia data, to fit the mean velocities and all
three components of the intrinsic velocity dispersion tensor. The Milky Way is
the first galaxy for which all intrinsic phase space coordinates are available,
and the kinematics are superior to the best integral-field kinematics of
external galaxies. This situation removes the long-standing dynamical
degeneracies and makes this the first dynamical model highly over-constrained
by the kinematics. For these reasons, our ability to fit the data provides a
fundamental test for both galaxy dynamics and the mass distribution in the
Milky Way disc. We tightly constrain the volume average total density
logarithmic slope, in the radial range 3.6--12 kpc, to be $\alpha_{\rm
tot}=-2.149\pm 0.055$ and find that the dark halo slope must be significantly
steeper than $\alpha_{\rm DM}=-1$ (NFW). The dark halo shape is close to
spherical and its density is $\rho_{\rm DM}(R_\odot)=0.0115\pm0.0020$ M$_\odot$
pc$^{-3}$ ($0.437\pm0.076$ GeV cm$^{-3}$), in agreement with previous
estimates. The circular velocity at the solar position $v_{\rm circ}(R_{\odot})
= 236.5\pm 3.1$ km s$^{-1}$ (including systematics) and its gently declining
radial trends are also consistent with recent determinations.
