An Old, Metal-rich Accreted Stellar Component in the Milky Way Stellar Disk

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