Abstract

The Milky Way is the one galaxy where we can measure kinematics and precise intrinsic properties for large samples of individual stars. It is therefore an important Rosetta Stone for contextualizing our observations of all other galaxies and their stellar populations. It also provides a unique laboratory for studying up close the dynamical evolution of galaxies and the small-scale properties of dark matter. However, our view of the Galaxy is static: We do not see stellar orbits directly, as we only have access to instantaneous measurements of stellar positions and velocities. Much of what we have learned about the Milky Way — for example, its structural parameters and the distribution of dark matter — has therefore come from modeling our snapshot of the Galaxy under strong assumptions about its dynamical state, such as time-independence or equilibrium. Precise kinematic data from contemporary surveys like the Gaia mission are strongly challenging these assumptions: Significant signatures of time-dependence appear ubiquitously from the solar neighborhood, to the outer Galactic disk, to the orbits of stellar streams and stars in the stellar halo. I will discuss the implications of these revelations on our understanding of the Galaxy, and how explicitly modeling and exploiting this disequilibrium is leading to an even more precise understanding of the Milky Way and dark matter.