Chronology of the chemical enrichment of the old Galactic stellar populations

TL;DR

This study uses precise ages and chemical abundances of old stars to reconstruct the Milky Way's merger history, revealing the timeline of Gaia-Enceladus's last major merger and its impact on Galactic evolution.

Contribution

It provides high-precision ages and chemical data for old stellar populations, clarifying the chronological sequence of the Milky Way's accretion events and their effects on Galactic structure.

Findings

Gaia-Enceladus merger lasted at least 3 billion years.

The last major merger occurred approximately 9.6 billion years ago.

The merger influenced the formation of the Galactic disc.

Abstract

The Milky Way accreted several smaller satellite galaxies in its history. These mergers added stars and gas to the Galaxy and affected the properties of the pre-existing stellar populations. Stellar chemical abundances and ages are needed to establish the chronological order of events that occur before, during, and after such mergers. We report precise ages ($\sim$6.5%) and chemical abundances for the Titans, a sample of old metal-poor dwarfs and subgiants with accurate atmospheric parameters. We also obtain ages with an average precision of 10% for a selected sample of dwarf stars from the GALAH survey. We used these stars, located within $\sim$1 kiloparsec of the Sun, to analyse the chronology of the chemical evolution of in-situ and accreted metal-poor stellar populations. We determined ages by isochrone fitting. For the Titans, we determined abundances of Mg, Si, Ca, Ti, Ni, Ba, and Eu using spectrum synthesis. The [Mg/Fe] abundances of the GALAH stars were re-scaled to be consistent with the abundances of the Titans. We separated stellar populations by primarily employing chemical abundances and orbits. We find that star formation in the so-called Gaia-Enceladus or Gaia-Sausage galaxy, the last major system to merge with the Milky Way, lasted at least 3 billion years and got truncated 9.6 $\pm$ 0.2 billion years ago. This marks with very high precision the last stage of its merging process. We also identified stars of a heated metal-poor in-situ population with virtually null net rotation, probably disturbed by several of the early Milky Way mergers. We show that this population is more metal rich than Gaia-Enceladus at any time. The sequence of events uncovered in our analysis supports the hypothesis that Gaia-Enceladus truncated the formation of the high-$\alpha$ disc and caused the gas infall that forms the low-$\alpha$ disc, in agreement with theoretical predictions.