Subproject A7: Unveiling Dark Matter and Fossil Records in the Solar Neighborhood

Inferring the local dark matter density is a challenge affected by, among other limitations, the still comparatively small stellar samples. A long-term goal of subproject A7 is to overcome these limitations in chemo-kinematical modeling for the extended Solar neighborhood. Excellent discrete data have become available over the past few years, but development of proper analysis and modeling methods has clearly been lagging behind. With our newly developed likelihood fitting approach, we have been able to show during the first funding period that the vertical and radial motions in the Milky Way disk are significantly coupled and robustly measure this so-called velocity ellipsoid tilt. This resulted in the problem that the commonly adopted modeling assumption of a zero tilt is invalid, but we succeeded in deriving and applying a new solution of the Jeans equations that allows for a non-zero tilt. These computationally-efficient Jeans models will make it possible to handle massive datasets such as from Gaia and to constrain the very large parameter range before applying more general but computationally-intensive modeling approaches like Schwarzschild's orbit-superposition method.

A self-consistent dynamical model fitted to these observational data from RAVE, SDSS, the Gaia-ESO Survey, and soon Gaia data can connect the Galactic gravitational potential and the phase-space distribution functions of the stars selected by abundance patterns. With accurate measurements of the gravitational potential and precise constraints on the stellar orbit distribution, we can address the following central issues in the second funding period: What is the local density of dark matter, needed to inform laboratory experiments designed to detect dark matter particles and to constrain the overall dark matter distribution in the Milky Way? Which fraction of stars at the solar radius has come from disrupted satellites and which fraction is trapped in dynamical resonances? This project will combine a likelihood approach with Schwarzschild’s orbit-superposition method to derive consistently the gravitational potential and the phase space distribution functions of stellar subpopulations dependent on their abundance pattern. We can address important questions such as: What is the local dark matter density and which stars originate from disrupted satellite galaxies?