Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood

TL;DR

This study measures the mass density in the solar neighborhood using thick disk star kinematics and finds no evidence for dark matter, challenging standard Galactic dark matter models and suggesting a need to reconsider dark matter distribution.

Contribution

The paper provides the first direct dynamical measurement of the local mass density that strongly constrains the presence of dark matter in the solar neighborhood.

Findings

No dark matter component needed to explain the observed mass density

Spherical dark matter halo models are excluded at >4 sigma confidence

Only highly prolate dark matter halos could fit the observations, which are unlikely in standard models

Abstract

We estimated the dynamical surface mass density Sigma at the solar position between Z=1.5 and 4 kpc from the Galactic plane, as inferred from the kinematics of thick disk stars. The formulation is exact within the limit of validity of a few basic assumptions. The resulting trend of Sigma(Z) matches the expectations of visible mass alone, and no dark component is required to account for the observations. We extrapolate a dark matter (DM) density in the solar neighborhood of 0+-1 mM_sun pc^-3, and all the current models of a spherical DM halo are excluded at a confidence level higher than 4sigma. A detailed analysis reveals that a small amount of DM is allowed in the volume under study by the change of some input parameter or hypothesis, but not enough to match the expectations of the models, except under an exotic combination of non-standard assumptions. Identical results are obtained when repeating the calculation with kinematical measurements available in the literature. We demonstrate that a DM halo would be detected by our method, and therefore the results have no straightforward interpretation. Only the presence of a highly prolate (flattening q>2) DM halo can be reconciled with the observations, but this is highly unlikely in LambdaCDM models. The results challenge the current understanding of the spatial distribution and nature of the Galactic DM. In particular, our results may indicate that any direct DM detection experiment is doomed to fail, if the local density of the target particles is negligible.