Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations

TL;DR

This study identifies two chemically and kinematically distinct metal-poor stellar populations in the Galactic halo using APOGEE data, revealing their likely different origins as in situ and accreted stars.

Contribution

It provides a detailed chemical and kinematic analysis of metal-poor stars, distinguishing two populations and linking them to different formation histories.

Findings

Two chemically distinct populations identified in [Fe/H] - [Mg/Fe] plane.

Low-Mg population shows halo-like kinematics, likely accreted.

High-Mg population exhibits thick disk-like chemistry and rotation.

Abstract

We find two chemically distinct populations separated relatively cleanly in the [Fe/H] - [Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] $< -0.9$) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the $\alpha$-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote the HMg population) exhibits a significant net Galactic rotation, whereas the low-Mg population (or LMg population) has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely as an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with $\it in$ $\it situ$ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low- and high-$\alpha$ halo stars found in previous studies, suggesting that these are samples of the same two populations.