Age Determination of Galaxy Merger Remnant Stars using Asteroseismology

TL;DR

This study uses asteroseismology combined with Gaia and APOGEE data to precisely determine the ages of stars originating from past dwarf galaxy mergers with the Milky Way, shedding light on the Galaxy's merger history.

Contribution

It introduces a method to accurately date ex-situ stars from galaxy mergers, improving understanding of the timing of these events in the Milky Way's evolution.

Findings

Ex-situ stars are older than 8 Gyr.

Mean age of Gaia-Enceladus/Sausage stars is approximately 9.5 Gyr.

Merger event associated with Gaia-Enceladus occurred 8-10 Gyr ago.

Abstract

The Milky Way was shaped by the mergers with several galaxies in the past. We search for remnant stars that were born in these foreign galaxies and assess their ages in an effort to put upper limits on the merger times and thereby better understand the evolutionary history of our Galaxy. Using 6D-phase space information from Gaia eDR3 and chemical information from APOGEE DR16, we kinematically and chemically select $23$ red giant stars belonging to former dwarf galaxies that merged with the Milky Way. With added asteroseismology from Kepler and K2, we determine the ages of the $23$ ex-situ stars and $55$ in-situ stars with great precision. We find that all the ex-situ stars are consistent with being older than $8$ Gyr. While it is not possible to associate all the stars with a specific dwarf galaxy we classify eight of them as Gaia-Enceladus/Sausage stars, which is one of the most massive mergers in our Galaxy's history. We determine their mean age to be $9.5^{+1.2}_{-1.3}$ Gyr consistent with a merger time of $8$-$10$ Gyr ago. The rest of the stars are possibly associated with Kraken, Thamnos, Sequoia, or another extragalactic progenitor. The age determination of ex-situ stars paves the way to more accurately pinning down when the merger events occurred and hence provide tight constraints useful for simulating how these events unfolded.