The metal-rich halo tail extended in |z|: a characterization with Gaia DR2 and APOGEE

Abstract

We report an analysis of the metal-rich tail ([Fe/H]>−0.75) of stars located at distances fromthe Galactic plane up to|z|∼10 kpc, observed by the Apache Point Observatory GalacticEvolution Experiment (APOGEE). We examine the chemistry, kinematics, and dynamicsof this metal-rich sample using chemical abundances and radial velocities provided by the14th APOGEE data release (DR14) and proper motions from the secondGaiadata release(DR2). The analysis reveals three chemically different stellar populations in the [Mg/Fe]versus[Fe/H]space–ahigh-[Mg/Fe]andlow-[Mg/Fe]populations, and a third group withintermediate [Mg/Fe]∼+0.1 – as well as for other chemical elements. We find that they arealso kinematically and dynamically distinct. The high-[Mg/Fe] population exhibits a prograderotation which decreases down to 0 as|zmax|increases, as well as eccentric orbits that aremore bound and closer to the plane. The low-[Mg/Fe] stars are likely Sagittarius members,moving in less-bound orbits reaching larger distances from the centre and the Galactic plane.The intermediate-[Mg/Fe] stars resembles the two stellar overdensities lying about|z|∼5 kpcrecently reported in the literature, for which a disc origin has been claimed. We report theidentification of new members of these two disc-heated overdensities