A new determination of the local dark matter density from the kinematics of K dwarfs

Abstract

We apply a new method to determine the local disc matter and dark halo matter density to kinematic and position data for \sim2000 K dwarf stars taken from the literature. Our method assumes only that the disc is locally in dynamical equilibrium, and that the 'tilt' term in the Jeans equations is small up to \sim1 kpc above the plane. We present a new calculation of the photometric distances to the K dwarf stars, and use a Monte Carlo Markov Chain to marginalise over uncertainties in both the baryonic mass distribution, and the velocity and distance errors for each individual star. We perform a series of tests to demonstrate that our results are insensitive to plausible systematic errors in our distance calibration, and we show that our method recovers the correct answer from a dynamically evolved N-body simulation of the Milky Way. We find a local dark matter density of {\rho}dm = 0.025+0.014-0.013 M\odotpc^{-3} (0.95+0.53-0.49 GeV cm^{-3}) at 90% confidence assuming no correction for the non-flatness of the local rotation curve, and {\rho}dm = 0.022+0.015-0.013 M\odotpc^-3 (0.85+0.57-0.50 GeV cm^{-3}) if the correction is included. Our 90% lower bound on {\rho}dm is larger than the canonical value typically assumed in the literature, and is at mild tension with extrapolations from the rotation curve that assume a spherical halo. Our result can be explained by a larger normalisation for the local Milky Way rotation curve, an oblate dark matter halo, a local disc of dark matter, or some combination of these.