The Rapid Formation of the Metal Poor Milky Way

TL;DR

This study uses stellar ages and chemical data from the H3 survey to reconstruct the assembly timeline of the Milky Way, revealing rapid formation and merger events within the first 10 billion years.

Contribution

It provides new age estimates for multiple galactic substructures, refining the timeline of the Milky Way's early assembly and merger history.

Findings

GSE merged with the Milky Way about 9.5-10.2 Gyr ago.

Other mergers occurred between 9-13 Gyr ago.

The metal-poor disk built up rapidly within a few billion years.

Abstract

Our understanding of the assembly timeline of the Milky Way has been transforming along with the dramatic increase in astrometric and spectroscopic data available over the past several years. Many substructures in chemo-dynamical space have been discovered and identified as the remnants of various galactic mergers. To investigate the timeline of these mergers we select main sequence turn off & subgiant stars (MSTOs) from the H3 survey, finding members in seven metal poor components of the halo: GSE, the Helmi Streams, Thamnos, Sequoia, Wukong/LMS-1, Arjuna, and I'itoi. We also select out the metal poor in situ disk to facilitate comparison to the evolution of the Milky Way itself at these early epochs. We fit individual isochrone ages to the MSTOs in each of these substructures and use the resulting age distributions to infer simple star formation histories. For GSE we resolve an extended star formation history that truncates $\approx10$ Gyr ago, as well as a clear age -- metallicity relation. From this age distribution and measured star formation history we infer that GSE merged with the Milky Way at a time $9.5-10.2$ Gyr ago, in agreement with previous estimates. We infer that the other mergers occurred at various times ranging from $9-13$ Gyr ago, and that the metal poor component of the disk built up within only a few billion years. These results reinforce the emerging picture that both the disk and halo of the Milky Way experienced a rapid assembly.