Deciphering the Milky Way disc formation time encrypted in the bar chrono-kinematics

TL;DR

This paper introduces a new method using the kinematic signatures of Mira variables to estimate the Milky Way disc's formation time, constraining it to be younger than about 11-12 billion years, based on the inner Galaxy's dynamics.

Contribution

The study presents a novel approach to determine the Milky Way's disc formation epoch through inner Galaxy kinematics and Mira variable analysis, independent of solar vicinity data.

Findings

The oldest stars with bar imprints match the disc spin-up epoch.

Bar kinematic signatures vanish for Mira variables with periods under 190 days.

The Milky Way's spin-up epoch is constrained to be younger than ~11-12 Gyr.

Abstract

We present a novel method to constrain the formation time of the Milky Way disc using the chrono-kinematic signatures of the inner Galaxy. We construct an O-rich Mira variable sample from the Gaia Long-period Variable catalogue to study the kinematic behaviour of stars with different ages in the inner Galaxy. From the Auriga suite of cosmological zoom-in simulations, we find that the age of the oldest stellar population with imprints of the bar in density and kinematics matches the disc spin-up epoch. This is because stars born before the spin-up show insufficient rotation and are not kinematically cold enough to be efficiently trapped by the bar. We find that the bar kinematic signature disappears for Mira variables with a period shorter than 190 days. Using the period-age relation of Mira variables, we constrain the spin-up epoch of the Milky Way to be younger than $\sim11-12$~Gyr (redshift $\sim3$). We also discuss and compare our method and result to other evidence of the Milky Way spin-up epoch under the context of a realistic age uncertainty. Age uncertainty leads to an overestimation of the disc formation time when performing backward modelling. Our constrain of the spin-up epoch is independent from previous studies because it relies on the kinematics of the inner Galaxy instead of the solar vicinity.