Abstract
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 overconstrained 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 αtot = −2.149 ± 0.055 and find that the dark halo slope must be significantly steeper than αDM = −1 (NFW). The dark halo shape is close to spherical and its density is ρDM(R⊙) = 0.0115 ± 0.0020 M⊙ pc−3 (0.437 ± 0.076 GeV cm−3), in agreement with previous estimates. The circular velocity at the solar position vcirc(R⊙) = 236.5 ± 3.1 km s−1 (including systematics) and its gently declining radial trends are also consistent with recent determinations.
Highlights
For decades astrophysicist have constructed dynamical models of external galaxies from observations of their unresolved stellar kinematics to study their masses as well as their orbital and density distributions
We provide a description of the mass density distribution of the Milky Way at radii r ≈ 3.6–12 kpc
The first component one needs to construct for a stellar dynamical model is the distribution of the tracer population, from which the kinematics was derived
Summary
For decades astrophysicist have constructed dynamical models of external galaxies from observations of their unresolved stellar kinematics to study their masses as well as their orbital and density distributions (see review by Courteau et al 2014). When using only projected line-of-sight kinematics and the first two velocity moments, there are well-known fundamental degeneracies between the mass density and the orbital distribution, or anisotropy, for spherical galaxies. This mass–anisotropy degeneracy (Binney & Mamon 1982; Gerhard 1993) led to the development of techniques to extract the elusive shape of the stellar line-of-sight velocity distribution from the galaxy spectra Dynamical degeneracies are observed even with state-of-the-art dynamical models and data (Krajnovicet al. 2005; de Lorenzi et al 2009)
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