Seeing the Milky Way Disk Evolve

Disk galaxies present a great regularity in their stellar bodies, but reveal complex structures in their young stars and gas. What connects the highly structured birth conditions to the overall regularity of disk galaxies? To tackle this question, we can use the Milky Way, for which we have star-by-star data, as a model organism to study the processes setting the evolution of its disk. The recent technical advances in spectroscopic and astrometric surveys have made it possible to collect 6D phase-space information, abundances and ages for a large number of stars. I will present a statistical model for the evolution of the Milky Way disk, describing when and where its stars were born, with what metallicity, and how their orbits subsequently evolved until their present state. APOGEE and Gaia data constrain that model well and implies strong redistribution of stellar orbits: over the age of the disk, stars typically migrate a distance of about the disk’s scale length. This extensive migration seems to happen with only modest radial orbit heating. Consequently, the disk looks very regular without totally losing its radial gradients.