Exploring the origin of low-metallicity stars in Milky Way-like galaxies with the NIHAO-UHD simulations

TL;DR

This study uses high-resolution cosmological simulations to investigate the origins of low-metallicity stars in Milky Way-like galaxies, revealing their distribution, formation history, and potential as tracers of early galaxy assembly.

Contribution

It provides new insights into the origins and kinematics of low-metallicity stars, especially their confinement to the disk plane and their link to early galaxy formation phases.

Findings

Over 90% of retrograde low-metallicity stars are from early galaxy build-up.

Low-metallicity stars are often confined to the disk plane, similar to Milky Way observations.

Milky Way shows a large population of eccentric, very metal-poor planar stars, rare in simulations.

Abstract

The kinematics of the most metal-poor stars provide a window into the early formation and accretion history of the Milky Way. Here, we use 5~high-resolution cosmological zoom-in simulations ($\sim~5\times10^6$ star particles) of Milky Way-like galaxies taken from the NIHAO-UHD project, to investigate the origin of low-metallicity stars ([Fe/H]$\leq-2.5$). The simulations show a prominent population of low-metallicity stars confined to the disk plane, as recently discovered in the Milky Way. The ubiquity of this finding suggests that the Milky Way is not unique in this respect. Independently of the accretion history, we find that $\gtrsim~90$ per cent of the retrograde stars in this population are brought in during the initial build-up of the galaxies during the first few Gyrs after the Big Bang. Our results therefore highlight the great potential of the retrograde population as a tracer of the early build-up of the Milky Way. The prograde planar population, on the other hand, is accreted during the later assembly phase and samples the full galactic accretion history. In case of a quiet accretion history, this prograde population is mainly brought in during the first half of cosmic evolution ($t\lesssim7$~Gyr), while, in the case of an on-going active accretion history, later mergers on prograde orbits are also able to contribute to this population. Finally, we note that the Milky Way shows a rather large population of eccentric, very metal-poor planar stars. This is a feature not seen in most of our simulations, with the exception of one simulation with an exceptionally active early building phase.