The oldest and most metal poor stars in the APOSTLE Local Group simulations

TL;DR

This study uses cosmological simulations to analyze the distribution, formation history, and environmental dependence of the oldest and most metal-poor stars in Milky Way-sized galaxies within the Local Group.

Contribution

It provides new insights into the spatial distribution and formation timelines of metal-poor stars, highlighting their prevalence outside the solar circle and in dwarf galaxies.

Findings

Metal-poor stars increase in fraction with radius from galaxy center.

Half of the metal-poor stars formed after redshift 5.3.

Over half of the oldest and most metal-poor stars are found outside the solar circle.

Abstract

We examine the spatial distribution of the oldest and most metal poor stellar populations of Milky Way-sized galaxies using the APOSTLE cosmological hydrodynamical simulations of the Local Group. In agreement with earlier work, we find strong radial gradients in the fraction of the oldest (tform < 0.8 Gyr) and most metal poor ([Fe/H]< -2.5) stars, both of which increase outwards. The most metal poor stars form over an extended period of time; half of them form after z = 5.3, and the last 10% after z = 2.8. The age of the metal poor stellar population also shows significant variation with environment; a high fraction of them are old in the galaxy's central regions and an even higher fraction in some individual dwarf galaxies, with substantial scatter from dwarf to dwarf. Overall, over half of the stars that belong to both the oldest and most metal-poor population are found outside the solar circle. Somewhat counter-intuitively, we find that dwarf galaxies with a large fraction of metal poor stars that are very old are systems where metal poor stars are relatively rare, but where a substantial old population is present. Our results provide guidance for interpreting the results of surveys designed to hunt for the earliest and most pristine stellar component of our Milky Way.