Presentation #201.01 in the session “Adrian Price-Whelan Plenary”.
The Milky Way is the one galaxy where we can measure kinematics and intrinsic properties for large samples of stars across a wide range of stellar parameters (mass, age, element abundances). Our Galaxy therefore provides an important laboratory to study galaxy growth and evolution and the small-scale properties of dark matter. However, our view of the Galaxy is static: We do not see stellar orbits or stellar evolution directly, as we only have access to instantaneous measurements of stellar positions, velocities, and properties. Much of what we have learned about the Milky Way — for example, its structural parameters, the distribution of dark matter, its merger history — has therefore come from modeling our snapshot of the Galaxy with strong assumptions, such as equilibrium stellar orbits or time-independence of the Galactic mass distribution. Precise kinematic data from contemporary surveys like the Gaia mission are challenging these assumptions: Significant time-dependent phenomena 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 will lead to an even more precise understanding of the Milky Way.