Measuring the local dark matter density with LAMOST DR5 and Gaia DR2

Abstract

We apply the vertical Jeans equation to the kinematics of Milky Way stars in the solar neighbourhood to measure the local dark matter density. More than 90,000 G-and K-type dwarf stars are selected from the cross-matched sample of LAMOST DR5 and Gaia DR2 for our analyses. The mass models applied consist of a single exponential stellar disc, a razor thin gas disc and a constant dark matter density. We first consider the simplified vertical Jeans equation which ignores the tilt term and assumes a flat rotation curve. Under a Gaussian prior on the total stellar surface density, the local dark matter density inferred from Markov Chain Monte Carlo simulations is 0.0133 +0.0024 −0.0022 M pc −3 . The local dark matter densities for subsamples in an azimuthal angle range of −10 • < φ < 5 • are consistent within their 1σ errors. However, the northern and southern subsamples show a large discrepancy due to plateaux in the northern and southern vertical velocity dispersion profiles. These plateaux may be the cause of the different estimates of the dark matter density between the north and south. Taking the tilt term into account has little effect on the parameter estimations and does not explain the north and south asymmetry. Taking half of the difference of σ z profiles as unknown systematic errors, we then obtain consistent measurements for the northern and southern subsamples. We discuss the influence of the vertical data range, the scale height of the tracer population, the vertical distribution of stars and the sample size on the uncertainty of the determination of the local dark matter density.

Jupiter missions as probes of dark matter

“…where n χ is the local DM number density (we take the local DM energy density to be 0.4 GeV/cm 3 [53][54][55][56][57][58]), vχ 267.2 km/s is DM velocity dispersion [59], v J (r) is the escape velocity at a distance r from Jupiter's center and is v J (R J ), m n is the nucleon mass, n n (r)…”

Estimating the local dark matter density in a non-axisymmetric wobbling disc

“…Most work to date on estimating ρDM, has employed something akin to the 1D approximation (e.g. Read 2014;Xia et al 2016;Sivertsson et al 2018;Guo et al 2020;Salomon et al 2020). Here, we will also use this approximation as our default method.…”

“…Finally, the purple datapoint shows a recent determination from Widmark et al (2021) that is distinct from the other local methods in that it harnesses the fact that the Milky Way disc is wobbling and ringing. (Data taken from Xia et al (2016); Sivertsson et al (2018); Buch et al (2019); Guo et al (2020); Salomon et al (2020); Widmark et al (2021).). Schutz et al 2018;Buch et al 2019;Widmark 2019).…”

“…'Local' methods use Jeans and/or distribution function models over small volumes near the Sun (e.g. Xia et al 2016;Sivertsson et al 2018;Guo et al 2020; Salomon et al 2020). 'Global' methods use stellar tracers over larger volumes (e.g.…”

“…Some studies exploit the vertical kinematics of stars up to a height z ∼ 1 − 3 kpc above the Galactic disc (e.g. Xia et al 2016;Sivertsson et al 2018;Guo et al 2020; Salomon et al 2020), while others use a similar methodology much closer to the plane (< 200 pc;…”