# On the likelihoods of finding very metal-poor (and old) stars in the Milky Way's disc, bulge, and halo

**Authors:** Diego Sotillo-Ramos, Maria Bergemann, Jennifer K. S. Friske, Annalisa Pillepich  
**arXiv:** [2307.14421](https://arxiv.org/abs/2307.14421)  
**Date:** 2023-07-26  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.14421v1/

## Key points

- Small number of very metal-poor stars with cold kinematics discovered in the Galactic disc and bulge
- Origins of these stars remain mysterious
- Study conducted using TNG50 cosmological simulation to select Milky Way analogs based on properties at redshift z=0
- Aimed to make predictions for detectability of very metal-poor stars in upcoming observational programs
- Increased sample size of MW analogs compared to previous works
- Paper organized into sections describing cosmological simulation, analyzing populations of metal-poor stars, exploring additional properties, and discussing implications for understanding the Galaxy's disc
- TNG50 cosmological simulation consists of a periodic cubic volume with dark matter particles and gas cells/stellar particles
- Insights provided into statistical properties within the volume accessible to next-generation spectroscopic surveys like 4MOST, WEAVE, and MOONS
- Approximately 20% of very metal-poor stars belong to the disk in all galaxies, reaching up to 30% in some analogs
- Around 50±10% of VMP disk stars are older than 12.5 Gyr and about 70±10% come from accreted satellites
- Significant fraction (approximately 70%) of VMP stars belongs to the halo with a median age of 12 Gyr
- Results confirm earlier findings from simulations and suggest substantial amounts of very-and extremely-metal poor stars in the stellar disc of the Milky Way

## Summary

In recent observational studies, a small number of very metal-poor stars with cold kinematics have been discovered in the Galactic disc and bulge. However, the origins of these stars remain mysterious. To gain more insight into this phenomenon, Sotillo-Ramos et al. conducted a study using the TNG50 cosmological simulation to select Milky Way (MW) analogs based on their properties at redshift z=0, including disky morphology, stellar mass, and local environment. The researchers aimed to make predictions for the detectability of very metal-poor (VMP) stars in upcoming observational programs. They significantly increased the sample size of MW analogs compared to previous works based on zoom-in simulations. The paper is organized as follows: In Section 2, the authors describe the TNG50 cosmological simulation and explain how they selected MW-like galaxies. Section 3 focuses on analyzing the populations of metal-poor stars across different morphological components in the simulated galaxies. The researchers explore additional properties such as stellar ages, [Mg/Fe] abundances, and origin. In Section 4, possible implications for our current understanding of the origin and composition of the Galaxy's disc are discussed. The TNG50 cosmological simulation used in this study consists of a periodic cubic volume with a side length of 51.7 comoving Mpc. It includes dark matter particles and gas cells, with uniform masses for dark matter particles and an average mass for gas cells/stellar particles. Star formation follows a specific model that transforms gas into star particles when certain density thresholds are exceeded. By analyzing temporal and chemical properties of components in simulated galaxies, Sotillo-Ramos et al provide insights into statistical properties within the volume accessible to next-generation spectroscopic surveys like 4MOST, WEAVE, and MOONS. The focus is particularly on the 4MOST 4MIDABLE-HR survey. The study's findings reveal that approximately 20% of very metal-poor stars in all galaxies belong to the disk, with some analogs reaching up to 30%. Around 50±10% of VMP disk stars are older than 12.5 Gyr, and about 70±10% of them come from accreted satellites. A significant fraction of VMP stars belongs to the halo (approximately 70%) and has a median age of 12 Gyr. These results confirm earlier findings from simulations of fewer individual galaxies and suggest that the stellar disc of the Milky Way is likely to host substantial amounts of very-and extremely-metal poor stars; these stars predominantly originating from external sources can also form within the disc itself supporting its existence as primordial Galactic disc . In conclusion, Sotillo Ramos et al.