New Insights into the Formation Time of the Milky Way Disc

A recent paper titled "Deciphering the Milky Way disc formation time encrypted in the chrono-kinematics of the bar" by Hanyuan Zhang and collaborators presents a new method for determining the formation time of the Milky Way disc. The authors utilized kinematic signatures from the inner Galaxy, focusing on a sample of O-rich Mira variables sourced from the Gaia Long-period Variable catalogue.

The study reveals that the oldest stellar population showing the effects of the Galactic bar corresponds to the epoch when the disc began to spin up. Specifically, the authors found that stars formed before this spin-up lacked sufficient rotation and were not kinematically cold enough to be effectively trapped by the bar structure. The research indicates that the kinematic signature of the bar is absent in Mira variables with periods shorter than 190 days.

By applying the period-age relation of these Mira variables, the authors estimate that the spin-up epoch of the Milky Way is younger than approximately 11 to 12 billion years, corresponding to a redshift of about 3. This finding is significant as it provides a new independent constraint on the timing of disc formation, contrasting with previous studies that primarily relied on data from the solar vicinity. The authors emphasize that age uncertainty can lead to overestimations of disc formation time when using backward modeling techniques.

This research contributes to our understanding of the Milky Way's evolution and the dynamics of its stellar populations, offering insights that could influence future studies in galactic astrophysics. The full paper can be accessed at arXiv:2408.16815.