An old, metal-rich accreted stellar component in the Milky Way stellar disk

TL;DR

This study uncovers an ancient, metal-rich stellar component in the Milky Way's disk, suggesting past merger events contributed to the disk's formation, based on elemental abundances and stellar kinematics.

Contribution

It identifies a previously unknown old, metal-rich accreted stellar population in the Milky Way's disk using combined Gaia and APOGEE data, revealing new insights into galaxy formation.

Findings

Discovered an old, metal-rich accreted stellar component in the disk.

Linked chemical signatures with disk kinematics of RR Lyrae stars.

Confirmed the age of the accreted stars as exclusively old.

Abstract

We study the possibility that the Milky Way's cool stellar disc includes mergers with ancient stars. Galaxies are understood to form in a hierarchical manner, where smaller (proto-)galaxies merge into larger ones. Stars in galaxies, like the Milky Way, contain in their motions and elemental abundances tracers of past events and can be used to disentangle merger remnants from stars that formed in the main galaxy. The merger history of the Milky Way is generally understood to be particularly easy to study in the stellar halo. The advent of the ESA astrometric satellite Gaia has enabled the detection of completely new structures in the halo such as the Gaia-Enceladus-Sausage. However, simulations also show that mergers may be important for the build-up of the cool stellar disks. Combining elemental abundances for 100 giant branch stars from APOGEE DR17 and astrometric data from Gaia we use elemental abundance ratios to find an hitherto unknown, old stellar component in the cool stellar disk in the Milky Way. We further identify a small sample of RR Lyrae variables with disk kinematics that also show the same chemical signature as the accreted red giant stars in the disk. These stars allows us to date the stars in the accreted component. We find that they are exclusively old.