Chemical Evolution in Hierarchical Models of Cosmic Structure II: The Formation of the Milky Way Stellar Halo and the Distribution of the Oldest Stars

TL;DR

This study uses chemodynamical models to explore the formation and chemical evolution of the Milky Way's stellar halo, revealing how ancient, metal-poor stars are distributed and how they can inform us about the early universe.

Contribution

It introduces new chemodynamical models that connect high-redshift star formation conditions with present-day stellar halo properties, aiding the study of the epoch of reionization.

Findings

Oldest stars are concentrated in the galaxy's center, especially in the bulge area.

Stars from z > 6 are likely to have [Fe/H] < -3 and be on tightly bound orbits.

A significant fraction of stars older than z > 15 are found in the halo's inner regions.

Abstract

This paper presents theoretical star formation and chemical enrichment histories for the stellar halo of the Milky Way based on new chemodynamical modeling. The goal of this study is to assess the extent to which metal-poor stars in the halo reflect the star formation conditions that occurred in halo progenitor galaxies at high redshift, before and during the epoch of reionization. Simple prescriptions that translate dark-matter halo mass into baryonic gas budgets and star formation histories yield models that resemble the observed Milky Way halo in its total stellar mass, metallicity distribution, and the luminosity function and chemical enrichment of dwarf satellite galaxies. These model halos in turn allow an exploration of how the populations of interest for probing the epoch of reionization are distributed in physical and phase space, and of how they are related to lower-redshift populations of the same metallicity. The fraction of stars dating from before a particular time or redshift depends strongly on radius within the galaxy, reflecting the "inside-out" growth of cold-dark-matter halos, and on metallicity, reflecting the general trend toward higher metallicity at later times. These results suggest that efforts to discover stars from z > 6 - 10 should select for stars with [Fe/H] <~ -3 and favor stars on more tightly bound orbits in the stellar halo, where the majority are from z > 10 and 15 - 40% are from z > 15. The oldest, most metal-poor stars - those most likely to reveal the chemical abundances of the first stars - are most common in the very center of the Galaxy's halo: they are in the bulge, but not of the bulge. These models have several implications for the larger project of constraining the properties of the first stars and galaxies using data from the local Universe.