First Gaia dynamical model of the Milky Way disc with six phase space coordinates: a test for galaxy dynamics. (arXiv:1909.05269v1 [astro-ph.GA])

<a href="http://arxiv.org/find/astro-ph/1/au:+Nitschai_M/0/1/0/all/0/1">Maria Selina Nitschai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cappellari_M/0/1/0/all/0/1">Michele Cappellari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Neumayer_N/0/1/0/all/0/1">Nadine Neumayer</a>

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 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 average total density logarithmic slope, in the radial

range 3.6–12 kpc, to be $alpha_{rm tot}=-2.149pm 0.055$ 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)=0.0115pm0.0020$ M$_odot$ pc$^{-3}$ ($0.437pm0.076$ GeV

cm$^{-3}$), in agreement with previous estimates. The circular velocity at the

solar position $v_{rm circ}left(R_{odot}right) = 236.5pm 3.1$ km s$^{-1}$

(including systematics) and its radial trends are also consistent with recent

determinations.

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 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 average total density logarithmic slope, in the radial

range 3.6–12 kpc, to be $alpha_{rm tot}=-2.149pm 0.055$ 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)=0.0115pm0.0020$ M$_odot$ pc$^{-3}$ ($0.437pm0.076$ GeV

cm$^{-3}$), in agreement with previous estimates. The circular velocity at the

solar position $v_{rm circ}left(R_{odot}right) = 236.5pm 3.1$ km s$^{-1}$

(including systematics) and its radial trends are also consistent with recent

determinations.

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