Gaia-DR2 extended kinematical maps. Part III: Rotation curves analysis, dark matter, and Modified Newtonian dynamics tests

TL;DR

This study uses extended kinematic maps of the Milky Way to derive rotation curves and analyze the galaxy's mass distribution, testing dark matter and MOND models while considering deviations from equilibrium.

Contribution

It introduces a method to derive rotation curves from extended kinematic maps and assesses the impact of non-equilibrium conditions on mass estimates.

Findings

Rotation curves fit both dark matter and MOND models.

Deviations from equilibrium cause systematic errors in mass estimates.

Radial velocity deviations can lead to overestimation of galaxy mass.

Abstract

Recent statistical deconvolution methods have produced extended kinematical maps in a range of heliocentric distances that are a factor of two to three larger than those analysed in the Gaia Collaboration based on the same data. In this paper, we use such maps to derive the rotation curve both in the Galactic plane and in off-plane regions and to analyse the density distribution. By assuming stationary equilibrium and axisymmetry, we used the Jeans equation to derive the rotation curve. Then we fit it with density models that include both dark matter and predictions of the MOND (Modified Newtonian dynamics) theory. Since the Milky Way exhibits deviations from axisymmetry and equilibrium, we also considered corrections to the Jeans equation. To compute such corrections, we ran N-body experiments of mock disk galaxies where the departure from equilibrium becomes larger as a function of the distance from the centre. The rotation curve in the outer disk of the Milky Way that is constructed with the Jeans equation exhibits very low dependence on $R$ and $z$ and it is well-fitted both by dark matter halo and MOND models. The application of the Jeans equation for deriving the rotation curve, in the case of the systems that deviate from equilibrium and axisymmetry, introduces systematic errors that grow as a function of the amplitude of the average radial velocity. In the case of the Milky Way, we can observe that the amplitude of the radial velocity reaches $\sim 10\%$ that of the azimuthal one at $R\approx 20$ kpc. Based on this condition, using the rotation curve obtained from the Jeans equation to calculate the mass may overestimate its measurement.