The vertical force in the solar neighbourhood using red clump stars in TGAS and RAVE

Abstract

Aims. We investigate the kinematics of red clump (RC) stars in the solar neighbourhood by combining data from Tycho-Gaia Astrometric Solution (TGAS) and Radial Velocity Experiment (RAVE) to constrain the local dark matter density. Methods. After calibrating the absolute magnitude of RC stars, we characterized their velocity distribution over a radial distance range of 6−10 kpc and up to 1.5 kpc away from the Galactic plane. We then applied the axisymmetric Jeans equations on subsets representing the thin and thick disks to determine the (local) distribution of mass near the disk of our Galaxy. Results. Our kinematic maps are well behaved, permitting a straightforward local determination of the vertical force, which we find to be Kzthin = − 2454 ± 619 (km s−1)2 kpc−1 and Kzthick = − 2141 ± 774 (km s−1)2 kpc−1 at 1.5 kpc away from the Galactic plane for the thin and thick disk samples and for thin and thick disk scale heights of 0.28 kpc and 1.12 kpc, respectively. These measurements can be translated into a local dark matter density ρDM ~ 0.018 ± 0.002 M⊙ pc−3. The systematic error on this estimate is much larger than the quoted statistical error, since even a 10% difference in the scale height of the thin disk leads to a 30% change in the value of ρDM and a nearly equally good fit to the data.