Measuring the local matter density usingGaiaDR2

Abstract

Aims. We determine the total dynamical matter density in the solar neighbourhood using the second Gaia data release (DR2). Methods. The dynamical matter density distribution is inferred in a framework of a Bayesian hierarchical model, which accounts for position and velocity of all individual stars, as well as the full error covariance matrix of astrometric observables, in a joint fit of the vertical velocity distribution and stellar number density distribution. This was done for eight separate data samples, with different cuts in observed absolute magnitude, each containing about 25,000 stars. The model for the total matter density does not rely on any underlying baryonic model, although we assumed that it is symmetrical, smooth, and monotonically decreasing with distance from the mid-plane. Results. We infer a density distribution which is strongly peaked in the region close to the Galactic plane ( 60 pc), for all eight stellar samples. Assuming a baryonic model and a dark matter halo of constant density, this corresponds to a surplus surface density of approximately 5-9 M pc −2 . For the Sun's position and vertical velocity with respect to the Galactic plane, we infer Z = 4.76±2.27 pc and W = 7.24 ± 0.19 kms −1 . Conclusions. These results suggest a surplus of matter close to the Galactic plane, possibly explained by an underestimated density of cold gas. We discuss possible systematic effects that could bias our result, for example unmodelled non-equilibrium effects, and how to account for such effects in future extensions of this work.

On the estimation of the local dark matter density using the rotation curve of the Milky Way

“…The obtained values of ρ DM, from the analyses of the B1 and the B2 baryonic models are around ρ DM, ∼ 0.30 GeV/cm 3 and ρ DM, ∼ 0.38 GeV/cm 3 , respectively. These values are consistent with most of the previous studies (some recent examples are [16,[29][30][31][32]), but smaller than others (such as the recent estimates of [33,34]). One explanation could be the different method used to estimate ρ DM, , since [33,34] exploit the local z-Jeans equation method, which is based on the vertical movement of stars in a region close to the Solar System (see e.g.…”

Dark matter local density determination based on recent observations

“…These estimates are focused on a local volume that excludes the Galactic plane (for example, [16] analysed stars within a vertical distance of |z| = 515-1247 pc for their α-young population and |z| = 634-2266 pc for their α-old population). On the contrary, the three ρ DM, studies that used Gaia's information (references [17,18,19]) present larger uncertainties and some of their estimates do not agree that well with the three older studies. One important difference is that the new estimates are concentrated on the local |z| < 200 pc, including the Galactic plane.…”

“…Those studies based on the rotation curve ( [13,14,12]), the distribution function fitting ( [8,10] are presented in pink, blue and gray, respectively. The colour is darker for the analyses [17,18,19,12] to show the use of Gaia's observations. It is easy to see a clear difference in the precision and agreement of earlier ρ DM, estimates and those performed after 2017, especially in the three estimates that included Gaia's observations and applied the vertical Jeans equation method.…”

Implications of a Time-varying Galactic Potential for Determinations of the Dynamical Surface Density

“…This collision induces a vertical wobble in the disk and in-plane rings, features that are not accounted for in the estimate of the local matter density inferred from the observed kinematics of the stars. Our analysis is a simple zeroth-order one, as is often performed in the solar neighbourhood, without using nonparametric approaches or taking into account the possible effects of non-axisymmetry (Widmark & Monari 2019;Widmark 2019;Silverwood et al 2016).…”

“…Bovy & Rix 2013). Locally the dynamical surface mass density estimates inferred from stars oscillating vertically above and below the plane (Kuijken & Gilmore 1991;Garbari et al 2012;Bovy & Tremaine 2012;Bienaymé et al 2014;Hagen & Helmi 2018;Kuijken & Gilmore 1989a,b;Siebert et al 2003;Holmberg & Flynn 2004;Bovy & Tremaine 2012;Widmark & Monari 2019;Widmark 2019) have been used to probe the local dark matter density, the dark matter vertical distribution and its possible deviations from a spherical halo profile, as well as the presence of a dark disk, thereby providing potential constraints on the nature of dark matter, and on alternatives (Bienaymé et al 2009). In particular, if the dark matter density is very low at the solar vicinity, around the midplane, the relative contributions from visible and dark matter can be disentangled by trying to constrain the gravitational potential out to larger heights (Bovy & Tremaine 2012;Holmberg & Flynn 2004).…”