First Gaia dynamical model of the Milky Way disc with six phase space coordinates: a test for galaxy dynamics

TL;DR

This paper develops the first detailed dynamical model of the Milky Way disc using Gaia DR2 data, providing new constraints on the galaxy's mass distribution and dark matter halo shape.

Contribution

It introduces a comprehensive axisymmetric dynamical model with 6D phase-space data, overcoming previous degeneracies and enabling precise measurements of the Milky Way's mass profile.

Findings

Total density slope tightly constrained to -2.149±0.055

Dark halo density at solar position is 0.0115±0.0020 M_sun/pc^3

Circular velocity at solar position is 236.5±3.1 km/s

Abstract

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.