Where are the Population III star relics in the simulated Milky Way analogues?

TL;DR

This study uses high-resolution simulations of Milky Way-like galaxies to locate and characterize Population III star relics, revealing their distribution mainly in the stellar halo and satellite galaxies, aiding observational searches.

Contribution

First detailed analysis of Population III relics' distribution and kinematics in simulated Milky Way analogues across multiple resolutions.

Findings

Relics formed from redshift 22 to 4, averaging z~10.

Approximately 46% of relics are in the outer stellar halo.

Relics are most abundant in low-mass, early-formed satellite galaxies.

Abstract

Using 6 Milky Way analogues with two different numerical resolutions from the Auriga simulation, we investigate the total mass, spatial distribution and kinematics of the Population III star relics in the Milky Way analogues at $z=0$. These relics (primarily second generation stars) formed over a wide redshift range, from about $z=22$ to $z=4$, with an average formation redshift of $z \sim 10.0$, and comprise about $2\times10^{-5}$ of the entire galactic stellar population. The disk and bulge components host only a small fraction of these relics, contributing less than $12$ percent in total. The stellar halo, in particular the outer stellar halo of which galactic radius $r>30$ kpc, hosts the largest fraction (about 46 percent on average), with an average of one relic star for per $4,000$ to $10,000$ stars, making it a promising region for observational searches. Additionally, around $18$ percent of the Population III star relics are found in satellite galaxies, with smaller and older satellite galaxies tending to contain a higher proportion of these stars. Thus, low-mass and early-formed satellite galaxies are also ideal targets for finding such relics, although some satellite galaxies may lack them entirely. The spatial distribution and kinematics of these stars show good numerical convergence across different simulation resolutions. Our results provide valuable guidance for searches of the Population III star relics and offer insights for interpreting findings from ongoing and future stellar archaeology surveys.