Disentangling the Galactic Halo with APOGEE: II. Chemical and Star Formation Histories for the Two Distinct Populations

TL;DR

This study investigates two distinct stellar populations in the Galactic halo using APOGEE data, revealing differences in their star formation histories and initial mass functions based on chemical signatures.

Contribution

It provides new insights into the chemical and star-formation histories of halo populations over a broad metallicity range using simple chemical evolution models.

Findings

High-alpha population has a more intense, longer-lived star formation history.

High-alpha population exhibits a top-heavier initial mass function.

Differences support multiple formation pathways for the Galactic halo.

Abstract

The formation processes that led to the current Galactic stellar halo are still under debate. Previous studies have provided evidence for different stellar populations in terms of elemental abundances and kinematics, pointing to different chemical and star-formation histories. In the present work we explore, over a broader range in metallicity (-2.2 < [Fe/H] < -0.5), the two stellar populations detected in the first paper of this series from metal-poor stars in DR13 of the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We aim to infer signatures of the initial mass function (IMF) and the most APOGEE-reliable alpha-elements (O, Mg, Si and Ca). Using simple chemical-evolution models, for each population. Compared with the low-alpha population, we obtain a more intense and longer-lived SFH, and a top-heavier IMF for the high-alpha population.