How Different Star-Sorting Methods Change Our View of the Milky Way’s Discs
The paper examines how five different methods for separating Milky Way thin- and thick-disc stars, chemical, age-based, kinematic, and dynamical, lead to different measurements of the discs’ structures. Chemical and age selections give the cleanest separation, while motion-based methods mix the populations. Across all approaches, the thin disc flares with radius, the thick disc stays roughly constant in height, and the thin disc has a longer scale length.
The Ancient Roots of the Milky Way’s Disks: Evidence for Early Co-Formation Before a Galactic Collision
Borbolato et al. find that the Milky Way’s thin and thick disks began forming over 11 billion years ago, earlier than previously thought. Using stellar ages and chemistry from APOGEE, LAMOST, and Gaia data, they show both disks co-formed, challenging models that rely on a major merger to start thin disk formation. Instead, the Gaia-Sausage Enceladus event likely halted thick disk growth and boosted thin disk star formation.