The epoch of the Milky Way's bar formation: dynamical modelling of Mira variables in the nuclear stellar disc

TL;DR

This study uses dynamical modeling of Mira variables in the Milky Way's nuclear stellar disc to date the formation of the galactic bar to approximately 8 billion years ago, revealing insights into the galaxy's early evolution.

Contribution

It introduces a novel method of dating the Milky Way's bar formation by analyzing Mira variables' kinematics and period--age relation in the nuclear stellar disc.

Findings

Nuclear stellar disc formed around 8 Gyr ago.

Evidence for inside-out growth and dynamical heating.

Transition in kinematics indicates formation epoch.

Abstract

A key event in the history of the Milky Way is the formation of the bar. This event affects the subsequent structural and dynamical evolution of the entire Galaxy. When the bar formed, gas was likely rapidly funnelled to the centre of the Galaxy settling in a star-forming nuclear disc. The Milky Way bar formation can then be dated by considering the oldest stars in the formed nuclear stellar disc. In this highly obscured and crowded region, reliable age tracers are limited, but bright, high-amplitude Mira variables make useful age indicators as they follow a period--age relation. We fit dynamical models to the proper motions of a sample of Mira variables in the Milky Way's nuclear stellar disc region. Weak evidence for inside-out growth and both radial and vertical dynamical heating with time of the nuclear stellar disc is presented suggesting the nuclear stellar disc is dynamically well-mixed. Furthermore, for Mira variables around a $\sim350$ day period, there is a clear transition from nuclear stellar disc-dominated kinematics to background bar-bulge-dominated kinematics. Using a Mira variable period-age relation calibrated in the solar neighbourhood, this suggests the nuclear stellar disc formed in a significant burst in star formation $(8\pm 1)\,\mathrm{Gyr}$ ago, although the data are also weakly consistent with a more gradual formation of the nuclear stellar disc at even earlier epochs. This implies a relatively early formation time for the Milky Way bar ($\gtrsim8\,\mathrm{Gyr}$), which has implications for the growth and state of the young Milky Way and its subsequent history.