JEANS ANALYSIS OF THE GALACTIC THICK DISK AND THE LOCAL DARK MATTER DENSITY

Abstract

ABSTRACT Dynamical estimates of the mass surface density at the solar radius can be made up to a height of 4 kpc using thick disk stars as tracers of the potential. We investigate why different Jeans estimators of the local surface density lead to puzzling and conflicting results. Using the Jeans equations, we compute the vertical (F z ) and radial (F R ) components of the gravitational force, as well as Γ(z), which is defined as Γ ≡ ∂ V c 2 / ∂ R ?> , with V c 2 ≡ − R F R ?> . If we assume that the thick disk does not flare and that all the components of the velocity dispersion tensor of the thick disk have a uniform radial scalelength of 3.5 kpc, Γ takes implausibly large negative values when using the currently available kinematical data of the thick disk. This implies that the input parameters or the model assumptions must be revised. We have explored, using a simulated thick disk, the impact of the assumption that the scalelengths of the density and velocity dispersions do not depend on the vertical height z above the midplane. In the absence of any information about how these scale radii depend on z, we define a different strategy. By using a parameterized Galactic potential, we find that acceptable fits to F z , F R , and Γ are obtained for a flaring thick disk and a spherical dark matter (DM) halo with a local density ≳0.0064 M ⊙ pc−3. Disk-like DM distributions may be also compatible with the current data of the thick disk. A precise measurement of Γ at the midplane could be very useful for discriminating between models.