Life in the fast lane: a direct view of the dynamics, formation, and evolution of the Milky Way's bar

Abstract: Studies of the ages, abundances, and motions of individual stars in the Milky Way provide one of the best ways to study the evolution of disk galaxies over cosmic time. The formation of the Milky Way's barred inner region in particular is a crucial piece of the puzzle of disk galaxy evolution. Using data from APOGEE and Gaia, we present maps of the kinematics, elemental abundances, and age of the Milky Way bulge and disk that show the barred structure of the inner Milky Way in unprecedented detail. The kinematic maps allow a direct, purely kinematic determination of the bar's pattern speed of 41+/-3 km/s/kpc and of its shape and radial profile. We find the bar's age, metallicity, and abundance ratios to be the same as those of the oldest stars in the disk that are formed in its turbulent beginnings, while stars in the bulge outside of the bar are younger and more metal-rich. This implies that the bar likely formed ~8 Gyr ago, when the decrease in turbulence in the gas disk allowed a thin disk to form that quickly became bar-unstable. The bar's formation therefore stands as a crucial epoch in the evolution of the Milky Way, a picture that is in line with the evolutionary path that emerges from observations of the gas kinematics in external disk galaxies over the last ~10 Gyr.